Considering a multi-user Downlink scenario, this paper studies transmit antenna selection (AS) for uniform rectangular arrays (URA). A key contribution of this work is the investigation of the trade-off between AS and the array directivity (or gain), and its impact on spectral efficiency (SE) and energy efficiency (EE). In this context, to improve EE of the system, a directivity-aware transmit AS and power allocation problem is formulated and solved using a sequential approach. For AS, two schemes are proposed: Block selection (BS) and L´evy flight based binary particle swarm optimization (LFBPSO), while for power allocation, an iterative quadratic transform method is employed in combination with Dinkelbach algorithm for EE maximization. The performance of the AS schemes is evaluated with respect to SE, EE and computational complexity. The results show that the array directivity varies with the subset of selected antennas and has a direct impact on the performance of the AS schemes in terms of achievable SE and EE. Furthermore, AS without considering the directivity can incur a loss in the array gain of up to 2 dB in a λ/2 spaced array. The results also show that LFBPSO outperforms other AS schemes with respect to SE and EE. Finally, another important contribution of this work is the experimental validation of the impact of array directivity on the AS process. This is demonstrated by replacing the isotropic antenna array with a measured 4×8 uniform rectangular array of patch antennas in the simulation framework.

This paper proposes a closed-form low-complexity method for RIS configuration in a multi-user cell-free massive MIMO (maMIMO) network. Leveraging the large-scale fading (LSF) distribution and spatial diversity of the distributed network architecture, the proposed method jointly enhances the end-to-end channel gain and the degrees of freedom for interference suppression. An LSF-based algorithm is introduced to maintain fairness among user equipment (UEs). We have analysed the proposed method under short-term and long-term RIS configuration schemes and different CSI acquisition strategies. A closed-form channel estimation error variance for$b$-bit digital RIS is derived, and the non-negligible impact of digital RIS on pilot contamination is quantified. To explore different trade-offs between spectral efficiency (SE) maximisation and fairness, we also propose an element-selection based RIS configuration method. To verify the performance, we derive three achievable SE equations for perfect CSI and imperfect CSI scenarios with a fully centralised cell-free maMIMO operation. Numerical results show that the proposed schemes outperform the baseline under three linear uplink combining methods and non-linear successive interference cancellation (SIC).

Reconfigurable Intelligent Surfaces (RIS) have emerged as a promising technology for enabling seamless wireless connectivity, particularly in environments characterized by non-line-of-sight (NLoS) propagation and signal blockages. To enhance performance in such scenarios, densely deployed RIS configurations is a promising solution. In this context, this paper investigates the impact of inter-element spacing on the gain and the energy efficiency (EE) of RIS-assisted communication system. Unlike previous works, we formulate the received signal model that incorporates the array directivity and show how the array gain varies with the change in the inter-element spacing. The performance of the standard (where the spacing is half of the wavelength) and dense RIS (where the spacing is less than half of the wavelength) configurations is compared in terms of, 1) beam pattern and array gain while considering quantized and unquantized phase shifts, 2) variation in the number of RIS elements, and 3) the trade-off between EE and array gain. Results reveal that while dense RIS offers slightly improved array gain compared to standard RIS, it incurs higher power consumption and reduced EE.

This paper introduces a compact Class-F/E power amplifier (PA) that eliminates the need for RF choke LRFCand incorporates a parallel resonant filter. The parallel resonant filter filters out the third harmonic of the switch current, thereby reducing the switch current stress on the PA. Additionally, design equations for calculating the component values are provided. To validate these equations, a prototype circuit was designed and simulated. At a operating frequency of 430 MHz, the simulated output power, drain efficiency, and gain of the prototype PA are 40.9 dBm, 79%, and 10.9 dB, respectively.

Uniform planar arrays (UPAs) are widely employed in practical systems, yet the study of antenna selection for UPAs remains limited. In particular, the interplay between selected antenna subsets and directivity has been overlooked in the existing literature. This paper addresses this by exploring how different subsets of selected antennas affect directivity and developing effective antenna selection strategies for UPA with a rectangular lattice. Our findings reveal that different subsets of selected antennas can lead to significantly different values for directivity, influencing selection outcomes compared to scenarios where directivity is not considered. Furthermore, we propose and compare four antenna selection strategies: 1) Column Selection (RS), 2) Row Selection (CS), 3) Full Aperture Greedy Selection (FAGS), and 4) Hybrid Initialization Binary Particle Swarm Optimization (HBPSO). With careful initialization of the particles, HBPSO achieves a spectral efficiency performance close to exhaustive search and surpasses other strategies. Moreover, incorporating directivity into the selection process significantly affects the performance of these strategies.

In this paper, we focus on the energy efficiency (EE) optimization and analysis of reconfigurable intelligent surface (RIS)-assisted multiuser downlink near-field communications. Specifically, we conduct a comprehensive study on several key factors affecting EE performance, including the number of RIS elements, the types of reconfigurable elements, reconfiguration resolutions, and the maximum transmit power. To accurately capture the power characteristics of RISs, we adopt more practical power consumption models for three commonly used reconfigurable elements in RISs: PIN diodes, varactor diodes, and radio frequency (RF) switches. These different elements may result in RIS systems exhibiting significantly different energy efficiencies (EEs), even when their spectral efficiencies (SEs) are similar. Considering discrete phases implemented at most RISs in practice, which makes their optimization NP-hard, we develop a nested alternating optimization framework to maximize EE, consisting of an outer integer-based optimization for discrete RIS phase reconfigurations and a nested non-convex optimization for continuous transmit power allocation within each iteration. Extensive comparisons with multiple benchmark schemes validate the effectiveness and efficiency of the proposed framework. Furthermore, based on the proposed optimization method, we analyze the EE performance of RISs across different key factors and identify the optimal RIS architecture yielding the highest EE.

The 3rd Generation Partnership Project (3GPP) continually endeavours to enhance spectral efficiency, reduce latency, refine protocols, and increase system security. These advancements have led to greater energy efficiency and resilience against attacks. However, the emergence of Open-RAN (O-RAN) and low-cost hardware capable of implementing the entire system have made spoofing attacks relatively easy to execute, often serving as a precursor to more significant security breaches. During cell selection, users are vulnerable until mutual trust is established through cryptographic means. Even after trust is established, UE-controlled mobility relies on cell re-selection based solely on signal strength, posing a potential weakness. To address this, a method is proposed to authenticate base stations at the physical layer by leveraging triple-lock characterization involving hardware, environment, and geographical location. This method, akin to Integrated Sensing and Communications (ISAC), facilitates binary classification (authentication) without necessitating characterisation of the range profile, modifications to the gNB or alterations to the physical layer signals. The proposed method achieves high classification accuracy (balanced accuracy), ranging from 94.4 (96.6)% to 97.2 (98.3)%, across various channel characteristics and base station hardware utilized in spoofing attacks. It distinguishes between transmitters experiencing similar environmental impacts and those with distinct environmental footprints. Importantly, the method achieves classification without computationally expensive training, exploiting cross-layer information and relying on modest ground truth/evaluated datasets collected over relatively brief periods. Furthermore, its suitability for deployment on portable devices enhances its practical applicability.

This paper presents a novel deep learning model for achieving millimeter accuracy for indoor localization. The model comprises a multi-head self-attention model and a convolutional neural network (CNN), allowing for robust feature extraction from the captured wireless signals. To further enhance the localization accuracy, we also introduced a data augmentation method to increase the size and diversity of the dataset by creating synthetic variants. The performance of the proposed model is tested on an open dataset containing measured channel state information (CSI) signals from a massive multiple-input multiple-output (MIMO) system. The validation accuracies for all three cases are more than seven times higher than the state of the art. The model has also been further evaluated in the COST INTERACT CA20120 Machine Learning Challenge. The performance of our model is competitive with the measured position and significantly outperforms other teams. The proposed model and the associated approaches contribute to the development of practical millimeter-level indoor localization systems using deep learning architectures.

Reconfigurable intelligent surface (RIS) is considered one of the most promising technologies for beyond 5G and 6G communications with its potential to redefine the propagation environment and the simplicity of implementation. This paper proposes a closed-form low-complexity scheme for RIS configuration in a distributed multi-user MIMO (MU-MIMO) network and introduces a joint initial access and grouping scheme for RIS assignment. By leveraging the eigenvalue distribution of the access point-side channel, the proposed method aligns signals at orthogonal subspaces and effectively increases the degree of freedom for interference suppression. To verify the performance, we derive two achievable spectral efficiency (SE) equations for centralised and distributed operations. Numerical results show that the proposed scheme outperforms the baseline obviously under three different uplink combining methods.

This study delves into the radiation pattern syn-thesis of reconfigurable intelligent surfaces (RIS) / reflection metasurfaces. Through superimposing multiple single-reflection profiles, which comprise the amplitude and/or phase settings of all constituent elements, a single incident wave can be effectively reflected in multiple asymmetric directions. However, some mismatch and interference between adjacent reflection beams may be caused by this superposition as well. Additionally, it is constrained by the inherent limitation that achieving linear and continuous amplitude adjustments and phase shifts in real-world designs is challenging. Consequently, the reconfigurable amplitude and phase must be approximated to discrete values, necessitating the arrangement of reflection profile before and after optimization based on integer. Therefore, in this paper, we adapt the traditional particle swarm optimization (PSO) algorithm to discretized integer-based PSO by proposing the concepts of ‘discard rate’ and ‘knowledge.’ With the enhancement of the integer-based programming, the multiple asymmetric reflection pattern can be synthesized with suppressed sidelobe levels within limited iterations and time cost.

In reconfigurable intelligent surface (RIS)-assisted wireless communication systems, the pointing accuracy and intensity of reflections depend crucially on the ’profile,’ representing the amplitude/phase state information of all elements in a RIS array. The superposition of multiple single-reflection profiles enables multi-reflection for distributed users. However, the optimization challenges from periodic element arrangements in single-reflection and multi-reflection profiles are understudied. The combination of periodical single-reflection profiles leads to amplitude/phase counteractions, affecting the performance of each reflection beam. This paper focuses on a dual-reflection optimization scenario and investigates the far-field performance deterioration caused by the misalignment of overlapped profiles. To address this issue, we introduce a novel deep reinforcement learning (DRL)-based optimization method. Comparative experiments against random and exhaustive searches demonstrate that our proposed DRL method outperforms both alternatives, achieving the shortest optimization time. Remarkably, our approach achieves a 1.2 dB gain in the reflection peak gain and a broader beam without any hardware modifications.

Cell-free massive multiple-input multiple-output (MIMO) is considered as a promising network architecture for beyond-5G communications with its potential of supporting the exponentially increasing mobile connections and resolving the inherent disadvantage of canonical cellular networks. This paper proposes a scalable pilot assignment algorithm for user-centric cell-free massive MIMO systems. The proposed scheme considers the eigenspace of channel vectors and minimizes the sum pilot contamination caused to all the serving APs of the currently considered UE. Interference-suppressing combining schemes are utilized in both centralized and distributed operations to evaluate the performance of the proposed algorithm. Furthermore, max-min SE fairness is considered by applying fractional power control. Numerical results show that the proposed scheme outperforms previous methods while providing the scalability to the network.

This paper characterises the impact of horizontally induced vibration on the antenna's channel response due to fast vehicle manoeuvres or the side push of wind during the overtaking of a large vehicle on a motorway at millimetre wave (mmWave) frequencies. The resultant misalignment perturbs the point of observation, which produces Doppler shifts. A new composite antenna and channel model, including horizontal vibration within the antenna mounting, is developed by measuring the scattering parameters (S21) at discrete intervals of time using a vector network analyser (VNA) that generates a swept tone with a radio frequency (RF) measurement bandwidth of 2GHz centred at 25.5GHz. The S21 over discrete intervals of time represents the channel time-frequency transfer function which is utilised to generate the Doppler variant transfer function and the Doppler power spectrum after post-processing at a vibration frequency below 100Hz. It is proven experimentally that the induced Doppler power is directly proportional to the probability density function (PDF) of the Doppler Shifts. This indicates that Doppler shifts with high probabilities of occurrence have higher Doppler powers. For example, a Doppler shift ≤2000Hz having a PDF = 0.6 or above arrives with a higher power of up to −20dBm as compared to the Doppler shift of ±4000Hz, which has a lower PDF = 0.1 and Doppler powers ≤−25dBm.

Autonomous vehicles require vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communication. Fifth-generation (5G) and beyond wireless networks enable such infrastructure through beamforming techniques at millimetre-wave (mm-wave) frequencies. This paper characterises the impact of antenna beamwidth on the Doppler shift, cross-polarisation ratio (XPR) and the channel attenuation for the 26GHz mm-wave V2I line-of-sight (LoS) channel. It is proven that the received Doppler power spectrum is proportional to the probability density function (PDF) of the Doppler Shift. This indicates that Doppler power reduces with the Doppler shift. It has also been established that a wider antenna half power beamwidth (HPBW) of 20° instead of 17° reduces the XPR by 20dB whilst increasing the channel attenuation by 10dB in a mm-wave LoS channel with dynamic blockage. Therefore, a trade-off between the XPR and channel attenuation can be introduced by employing different antenna HPBWs.

This paper investigates the effects of antenna vibration occurring due to suspension variation during the movement for vehicle-to-infrastructure (V2I) communication by developing a two dimensional (2D) model. With highly directional antennas fitted on a car's rear bumper and at a base station (BS), different beamwidths from 10° to 50° are investigated for a moving vehicle at 70mph. The gain sensitivity of the radiation pattern and the micro-Doppler shift are modelled for suspension variations between 5cm and 20cm. The analysis has been performed based on the observed angle of arrivals (AoA) at the receiver antenna in the elevation plane. The gain sensitivity represents the BSs’ main lobes' misalignments and receiver antennas’ radiation patterns at 26GHz. The results reveal a significant impact on gain sensitivity during the first 200m of movement away from the BS. Furthermore, the total Doppler shift increases due to the micro-Doppler with vertical displacements from the suspension variation. However, this can be mitigated if the beamwidth increases relative to the link budget or the BS height increases. The 2D model is validated by utilising accelerometer data recorded whilst the vehicle is moving at 70mph on a motorway, by extracting the displacements and computing the gain sensitivity.

Massive multiple-input, multiple-output (Ma-MIMO) is currently being deployed in the fifth generation (5G) networks. It has increased the capacity for sub-6GHz wireless access. However, the ongoing development is a critical step in enhancing this technology in order to deliver the capacity gains and the Quality of Service (QoS) requirements in 5G. Here, a novel spatial Software-Defined Networking (SDN) controller framework for radio resource allocation between a single cell long-term evolution (LTE) and a single cell Ma-MIMO is proposed and shown to further enhance performance. For the first time, an SDN-based radio resource management (SDN-RRM) framework for multiple radio access technologies (multi-RATs) is introduced. The methodologies in this framework address the interference caused by the user channel vectors as well as hardware impairments in Ma-MIMO by mainly relying on the Error Vector Magnitude (EVM) and channel state information (CSI). The handover between the LTE Access Point (AP) and the Ma-MIMO AP is decided by the wireless SDN controller. This has led to maximize the throughput and to achieve the QoS requirement. The proposed framework has been built and evaluated in real-time by using a massive Ma-MIMO testbed and an LTE testbed controlled by an SDN controller.

Millimeter wave (mmWave) communication has recently attracted significant attention from both industrial and academic communities. The large bandwidth availability as well as low interference nature of mmWave spectrum is particularly attractive for industrial communication. However, inherent challenges such as coverage and blockage of mmWave communication cause highly fluctuated channel quality. This paper explores wireless medium access control (MAC) schedulers for mmWave-based industrial wireless applications. Our objective is to design a high-performance and enhanced fairness MAC scheduling algorithm that responds rapidly to channel variations. The key contribution of our work is a method to modify the standard proportional fair (SPF) scheduler. It introduces more flexibility and dynamic properties. Compared to the SPF, our enhanced proportional fair (EPF) scheduler not only improves the priority for users in poor channel conditions but also accelerates the reaction time in fluctuated channel conditions. By providing higher fairness for all users and enhancing system robustness, it particularly adapts to the scatter-rich industrial mmWave communication environment. Through extensive performance evaluation based on the widely accepted network simulator (ns-3), we show that the new scheduler achieves better performance in terms of delivering ultra-low latency and reliable services over mmWave-based industrial communication.

Rate or link adaptation is the determination of the optimal modulation and coding scheme (MCS) that will maximize the performance under the current wireless channel conditions. A Software-Defined Networking (SDN) agent is a software element bridging an SDN controller and any legacy wireless network elements by providing the abstraction of these elements. In this paper, we present the work of an SDN approach for designing and implementing a Rate/Link Adaptation (RA) framework for wireless local area networks (WLAN). The framework provides support for real-time RA applications and flexibility to satisfy various degrees of Quality of Service (QoS) or Quality of Experience (QoE) requirements. We implement the proposed framework as an extension to the Wireless Open-Access Research Platform (WARP), an FPGA based Software-Defined Radio (SDR) platform, with evaluation results indicating the feasibility of using SDN-RA under the stringent time constraints posed by the WLAN. To demonstrate the effectiveness of decoupling rate decision functions from the underlying wireless interface card and to highlight its applicability for a diverse set of scenarios, we present a use case deployed over the framework focusing on rate adaptation for individual traffic, and display optimization in different aspects, such as the reduction transmission errors.

Signal to interference plus noise ratio (SINR) is a widely common performance metric used in the majority of massive multiple-input, multiple-output (Ma-MIMO) research. This metric requires prior knowledge of the user channel vectors and the interference caused by inaccurate channel state information (CSI). However, the interference caused by inaccurate CSI can't be calculated for real-world scenarios. On the other hand, a comprehensive performance indicator can be achieved by the Error Vector Magnitude (EVM) metric in real-world scenarios. This considers all impairments upon the transmitted symbol as seen at the receiver. However, measuring the EVM values for a subset of users requires each user to retransmit data symbols. This paper presents an estimation method with high accuracy by associating EVM to SINR values for Ma-MIMO with zero-forcing (ZF) and Minimum Mean Square Error (MMSE). Also introduced is a novel EVM prediction method for subset of users taken from the original set of simultaneous users in a single cell Ma-MIMO. This method jointly relies on the channel correlation between users and the EVM performance to predict the EVM values for a subset of the available users without the need to retransmit data symbols. This method considers the user channel vector and the interference caused by inaccurate CSI, which makes it suitable for Ma-MIMO algorithms, such as user grouping and power control. Real-world experimental data-sets with real-time results are carried out to validate the EVM prediction method using software-defined radio Ma-MIMO testbed.

The goals for 5G New Radio (NR) technology are to deliver wireless communications with higher capacity, performance, and efficiency compared with current mobile technologies. 5G NR will make effective use of the millimetre wave frequency spectrum to achieve these goals. However, these higher frequencies experience greater signal attenuation due to increased path loss and blockages. To overcome these, it is necessary to use high-gain directional active array antenna systems (AAS) at both ends of the communications link, which can steer the radiating beam in multiple directions. The cost-effective and compact implementation of the AAS requires the RF transceiver and antenna array to be highly integrated, making it impossible to independently test the performance of the constituent components due to the lack of inter-stage connectors. Further, the propagation channel is highly dynamic in both temporal and spatial domains. Hence, conformance testing of such systems is most effectively carried out using Over-The-Air (OTA) testing, since traditional conducted non-spatial test methods will not predict the radiated performance in space and the action of the RF transceiver, in particular, the beamformer. This paper presents a highly novel millimetre wave OTA test method facilitating the excitation of a Device Under Test (DUT) from multiple dynamic narrow angles of illumination, thus representing typical operating conditions and avoids the need for physical RF connections. This new method exploits the reflective properties of ellipsoidal surfaces and offers a cost-effective means when compared with other OTA test methods.

Recently, a lot of research is focused on applying Software-Defined Networking (SDN) concepts on wireless networks. However, the current wireless systems are lack of SDN support and remain closed and (mostly) proprietary, which makes it difficult to integrate with SDN systems. The SDN agent is introduced to solve the above issues. An SDN agent is a software element bridging the SDN controller and any legacy wireless network elements (NEs) by providing the abstraction of these elements. The main advantage of this approach is that there is no modification to either the existing SDN elements or the 802.11 protocols, while the SDN controller is enabled to control and manage legacy wireless NEs. In this paper, we present an SDN agent framework for WLANs and describe an implementation of the proposed agent on the Wireless Open-Access Research Platform (WARP), an FPGA based open source Software-Defined Radio (SDR) platform. With the support from such an agent, innovative applications and functionalities can be developed for the WLAN access points (APs), such as dynamically slicing AP bandwidth among users and adapting the transmit power in flexible granularity, i.e. at per frame/per flow level.

Fifth generation (5G) networks will utilise the large bandwidth available in millimetre wave (mmWave) frequency bands up to 100 GHz in order to meet the ever-increasing high data rate demands. Efficient communications system design for such networks requires a thorough understanding of the propagation properties of the electromagnetic signals at such high frequencies. This paper presents the results of a polarimetric channel measurement campaign at 26 GHz and 60 GHz showing that diffuse scattering can be a dominant propagation mechanism for mmWave links. The use of a wide bandwidth provides a detailed insight into the multipath structure of the channel. It is shown that in a diffuse scattering scenario, nearly co-incident signals create small scale fading that affects the flatness of the channel significantly even when directional antennas are used. This also results in large co-to-cross polarisation ratio (XPR) variations across the channel bandwidth.

This paper evaluates the impact of spatially multiplexing more users within a massive multiple- input, multiple-output (MIMO) system. Recent works on massive MIMO show that there is a peak value for sum spectrum efficiency (SE) achieved by serving a certain number of users. It was shown that until the sum SE reached its peak value, the maximum sum SE is achieved by serving all users simultaneously. These results were based on perfect channel state information (CSI) or Shannon capacity calculations or using very large number of antennas at the base station (BS). As opposed to the aforementioned results, we show that the maximum sum SE with practical number of antennas could be achieved by decreasing number of users before the sum SE reached its peak value. This is shown by calculating the sum SE based on the Error Vector Magnitude (EVM) performance. Sum SE with zero- forcing (ZF) and matched filtering (MF) is presented for uplink (UL) data transmission in a single-cell. Channel matrices formed from both independent and identically distributed (IID) Rayleigh samples and measured data from a real massive MIMO system were used for performance evaluation. The impact of adding more users is also demonstrated by real constellations captured from an indoor massive MIMO trail.

Enhanced network capacity and reduced overhead will be some of the future communication networks requirements. In the absence of Channel State Information (CSI) at the transmitter, Blind Interference Alignment (Blind IA) has shown that optimal Degrees of Freedom (DoF) can be achieved, minimizing network's overhead. Limited spectrum is a basic issue that millimeter-Wave (mmWave) frequencies can resolve in 5G systems. Our contribution is an interference management scheme, based on Blind IA that can be employed in multi-cell, slow-fading, mmWave networks. Blind IA manages inter-and intra-cell interference by antenna selection and appropriate message scheduling. We show that our proposed scheme achieves considerably higher sum rate than TDMA, and overall outperforms TDMA in terms of Degrees of Freedom (DoF) in most cases. Also, Blind IA provides better Bit Error Rate (BER) performance and fairness, in terms of scheduling, to the network users.

Responding to the unprecedented challenges imposed by the 5G technologies, mobile operators have given significant attention to Heterogeneous Cloud Radio Access Networks (H‐CRAN) due to their beneficial features of performing optimization, cost effectiveness, and improving spectral and energy efficiency performance. H‐CRAN inherits the attractive benefits of Heterogeneous Networks (HetNet) and the cloud computing by facilitating interference mitigation, scalability, and radio resource control. Consequently, H‐CRAN is proposed in this article as a cost‐effective potential solution to alleviate intertier interference and improve cooperative processing gains in HetNets by employing cloud computing. H‐CRAN can provide efficient resource sharing at the spectrum, network, and infrastructure levels. Therefore, this article proposes H‐CRAN cooperative interference mitigation method that enhances the time sharing among Radio Remote Heads (RRH) users. The study proposes an enhanced Almost Blank Subframe (ABSF) technique to increase the SINR and throughput of the small‐cell (low power base station) and macrocell users. Simulation results show that the proposed Dynamic P rogramming‐ D iverse A lmost Blank Subframe ( A BSF ) P attern ( DP-D A P) scheme improved the macro‐ and small‐cell users up to 56% and 35%, respectively, as compared to other state‐of‐the‐art ABSF schemes.

Deployment of low power base-stations (referred to as HetNet) within cellular systems can improve system capacity, coverage and enhance users experience. However, the inter-tier and intra-tier interference in such systems can significantly impact overall system performance, which makes interference and spectral resource management more complex. To build, operate HetNet networks, mobile operators have given significant attention to cloud radio access networks, C-RANs, due to their beneficial features of performance optimization and cost effectiveness. In C-RAN cooperated networks, the macro base-stations can operate cooperatively with small base-stations as advanced wireless access network paradigm, where cloud computing is used to fulfill the centralized large-scale cooperative processing. In this paper, such cooperated HetNet and C-RAN architecture is deployed to achieve effective spectral resources and interference management. The study proposes a user weighted probability-based algorithm, which divides the spectrum into shared and dedicated partitions, given that the amount of each partition is calculated efficiently based on the proposed methodology. The algorithm allocates each BS to the appropriate spectrum partition in order to meet certain QoS requirements and optimize overall users throughput in each tier.

Long Term Evolution (LTE) communications systems are increasingly being used for remote applications where a battery life of 10 to 15 years can be specified. To ensure these demands can be met, accurate modelling of power consumption is required. Existing models which decompose LTE power consumption focus on laboratory-only measurements and use continuous transmission scenarios. The work presented shows that whilst these are valid for the continuous case, they are not relevant for Machine to Machine (M2M) activity on a live network. Under these conditions, packet size is likely to be small in relation to protocol and system overheads, resulting in a dramatically increased and hard to model consumption of power. In addition, network measurement and optimisation transmissions in supposedly low power idle periods threaten to further confound the issue.

Millimeter wave (mmWave) communication is a key enabling technology with the potential to deliver high capacity, high peak data rate communications for future railway services. Knowledge of the radio characteristics is of paramount importance for the successful deployment of such systems. In this paper mmWave channel measurements are reported for a railway environment using a wideband channel sounder operating at 60GHz. Highly directional antennas are deployed at both ends of the link. Data is reported for path loss, root mean square (RMS) delay spread and K-factor. Static and mobile measurements are considered. Analysis shows that the signal strength is strongly dependent (up to 25dB) on the azimuth orientation of the directional transmit and receive antennas. A path loss exponent of n = 2. 04 was extracted from the Line-of-Sight measurements with optimally aligned antennas. RMS delay spreads ranged from 1ns to 22ns depending on antenna alignment. 50% of the measured K-factors were found to be less than 6dB. We conclude this is the result of ground reflections in the vertical Tx-Rx plane.

This paper presents the analysis and characterization of the surface scattering process for both specular and diffused components. The study is focused on the investigation of various building materials each having a different roughness, at a central frequency of 60GHz. Very large signal strength variations in first order scattered components is observed as the user moves over very short distances. This is due to the small-scale fading caused by rough surface scatterers. Furthermore, it is shown that the diffused scattering depends on the material roughness, the angle of incidence and the distance from the surface. Finally, results indicate that reflections from rough materials may suffer from high depolarization, a phenomenon that can potentially be exploited in order to improve the performance of mm-Wave systems using polarization diversity.

The first measured results for massive multiple-input, multiple-output (MIMO) performance in a line-of-sight (LOS) scenario with moderate mobility are presented, with 8 users served by a 100 antenna base Station (BS) at 3.7 GHz. When such a large number of channels dynamically change, the inherent propagation and processing delay has a critical relationship with the rate of change, as the use of outdated channel information can result in severe detection and precoding inaccuracies. For the downlink (DL) in particular, a time division duplex (TDD) configuration synonymous with massive MIMO deployments could mean only the uplink (UL) is usable in extreme cases. Therefore, it is of great interest to investigate the impact of mobility on massive MIMO performance and consider ways to combat the potential limitations. In a mobile scenario with moving cars and pedestrians, the correlation of the MIMO channel vector over time is inspected for vehicles moving up to 29km/h. For a 100 antenna system, it is found that the channel state information (CSI) update rate requirement may increase by 7 times when compared to an 8 antenna system, whilst the power control update rate could be decreased by at least 5 times relative to a single antenna system.

This paper proposes a novel uplink power control scheme in a single cell for massive MIMO (Multiple-Input-Multiple-Output). The proposed algorithm exploits the spatial degrees of freedom provided by massive MIMO to achieve the highest signal to interference-plus-noise ratio (SINR) for each user by considering the mutual spatial correlation arising from other users. It also minimizes the required uplink transmit power to achieve the highest possible uplink SINR per user without increasing the complexity at the user equipment. The base station (BS) controls the user transmit power using only two bits in a Transmission Power Control (TPC) command and the newly introduced algorithm is expected to increase both the energy efficiency (EE) and single cell spectral efficiency (SE) with reduced transmission overhead, latency and complexity in the receiver. Through recent preliminary trials, a 0.9 dB average SINR increase was achieved, and transmit power was decreased by up to 2.5 dB in the uplink. The proposed algorithm supports several popular linear MIMO decoders and precoders including Minimum Mean Square Error (MMSE), Zero-Forcing (ZF) and Matched Filtering (MF).

Ray tracing is a useful tool to model the spatial and temporal propagation parameters of electromagnetic radiation. The model can normally improve by the channel sounding. However, discrepancies may arise between the channel sounding and ray-tracing model due to it needs a wide bandwidth to differentiate the incoming signals, also the antenna radiation pattern may change dramatically over this wide frequency span. To overcome this limitation, in this paper, the sparse deconvolution will be used to test and improve data measurements from real environments. Good results are obtained that shows that Cauchy-Gaussian model can differentiate between two different signals that has 0.5m distances offset for 200 MHz and 0.8m for 150 MHz theoretically. Then, comparison between the measured data using the basic Inverse Discrete Fourier Transform and the Cauchy-Gaussian model for two different scenarios shows that signals arrival in time cannot adequately determined using only Inverse Discrete Fourier Transform technique at 150 MHz, while on the contrary, the Cauchy-Gaussian model can still discriminate between them.

Limited wireless spectrum in future networks is an issue that millimeter-Wave (mmWave) frequencies can resolve. Beamforming is a challenging operation for mmWave systems. Both analogue and hybrid solutions have been proposed. Apart from Orthogonal Multiple Access (OMA) systems, the Non-Orthogonal Multiple Access (NOMA) scheme has been recently considered as an alternative and can outperform OMA in many cases. Our contribution is the application of NOMA to a hybrid precoder design in mmWave indoor scenarios with multiple users. Introducing to our system appropriate user grouping and scheduling schemes, we show that the NOMA outperforms, in terms of spectral efficiency, OMA by providing a 17.1% improvement for high SNR values. Finally, applying NOMA with fair power allocation improves the rate of users with weak channel conditions by around 1 bps/Hz for SNR>5dB, without degrading the overall spectral efficiency in the cell.

Cloud Radio Access Network (C-RAN) has attracted a worldwide attention in both academia and industry. This network architecture re-forming has been considered as a potential solution to meet the increasing capacity demands for future mobile data traffic. In addition, Network Virtualisation is a promising technique for efficient resource utilisation. This paper proposes a customisable resource virtualisation algorithm for multi user data scheduling in a Long Term Evolution (LTE) C- RAN deployment. The algorithm is based on the hypervisor specific assignment of air resources allocation between the virtual operators (VOs) dynamically, based on either joint scheduling or per cell schemes. The objective is to improve the resource allocation mechanism based on traffic conditions and a database of pre-defined services priorities. Two distinctive scenarios are considered and evaluated against standard Round Robin (RR) C-RAN scheduling technique. Simulation results show improvements in the overall traffic throughput and reduction in end-to-end delay for delay sensitive applications. In addition, an assessment of fairness guarantee is considered across all users.

This paper examines the problem of non-line-of- sight (NLOS) identification and mitigation for geolocation signals in mobile networks. A ray tracing tool is used to simulate a mobile radio network with fixed base stations and thousands of mobile stations. The channel data between these mobile stations and base stations is used to extract parameters or features that are used for classification. Techniques for NLOS identification using a Least-Squares Support Vector Machine (LSSVM), are devised, producing greater than 98 percent accuracy for the proposed location specific approach, and 87 percent for the location independent approach. Respective LSSVM NLOS mitigation techniques are also proposed and evaluated. A usage context for the location specific approach is suggested, where the approach can help in addressing some of the challenges of next-generation wireless systems like massive MIMO.

This invited paper considers a key next step in the design of radio architectures aimed at supporting low energy consumption in 5G heterogeneous radio access networks. State-of-the-art mobile radios usually require one RF transceiver per standard, each working separately at any given time. Software defined radios, while spanning a wide range of standards and frequency bands, also work separately at any specific time. In 5G radio access networks, where continuous, multiband connectivity is envisaged, this conventional radio architecture results in high network power consumption. In this paper, we propose the novel concept of a concurrent multiband frequency-agile radio (CM-FARAD) architecture, which simultaneously supports multiple standards and frequency bands using a single, tunable transceiver. We discuss the subsystem radio design approaches for enabling the CM-FARAD architecture, including antennas, power amplifiers, low noise amplifiers and analogue to digital converters. A working prototype of a dual-band CM-FARAD test-bed is also presented together with measured salient performance characteristics.

The first measured results for massive multiple-input, multiple-output (MIMO) performance in a line-of-sight scenario with moderate mobility are presented, with eight users served in real time using a 100-antenna base station at 3.7 GHz. When such a large number of channels dynamically change, the inherent propagation and processing delay has a critical relationship with the rate of change, as the use of outdated channel information can result in severe detection and precoding inaccuracies. For the downlink (DL) in particular, a time-division duplex configuration synonymous with massive MIMO deployments could mean only the uplink (UL) is usable in extreme cases. Therefore, it is of great interest to investigate the impact of mobility on massive MIMO performance and consider ways to combat the potential limitations. In a mobile scenario with moving cars and pedestrians, the massive MIMO channel is sampled across many points in space to build a picture of the overall user orthogonality, and the impact of both azimuth and elevation array configurations are considered. Temporal analysis is also conducted for vehicles moving up to 29 km/h and real-time bit-error rates for both the UL and DL without power control are presented. For a 100-antenna system, it is found that the channel state information update rate requirement may increase by seven times when compared with an eight-antenna system, whilst the power control update rate could be decreased by at least five times relative to a single antenna system.

This paper presents initial results for a novel 128-antenna massive Multiple-Input, Multiple- Output (MIMO) testbed developed through Bristol Is Open in collaboration with National Instruments and Lund University. We believe that the results presented here validate the adoption of massive MIMO as a key enabling technology for 5G and pave the way for further pragmatic research by the massive MIMO community. The testbed operates in real-time with a Long-Term Evolution (LTE)-like PHY in Time Division Duplex (TDD) mode and supports up to 12 spatial streams, providing an excellent basis for comparison with existing standards and complimentary testbeds. Through line-of-sight (LOS) measurements at 3.51 GHz in an indoor atrium environment with 12 user clients, an uncoded system sum-rate of 1.59 Gbps was achieved in real-time using a single 20 MHz LTE band, equating to 79.4 bits/s/Hz. To the best of the author's knowledge, this is the highest spectral efficiency achieved for any wireless system to date.

The vast growth in mobile data traffic with its increasing capacity demands necessitates investigating future solutions to cope with these challenges. Network Virtualisation is considered one potential solution to simplify the current wireless networks. Recently efforts have been made to show the performance gains of different resource allocation schemes in legacy LTE air interface virtualization. Moreover Cloud - based mobile networks are anticipated to play a significant role for next generation mobile networks. This paper investigates the potential deployment benefits of a novel resource virtualization algorithm (Traffic Aware Joint Scheduling) in Cloud-RANs (C-RAN). Air interface resources are coordinated and allocated dynamically by a hypervisor among different virtual operators (VOs). Three distinctive schemes are proposed and evaluated against standard Round Robin (RR) C-RAN scheduling. Simulation results show improvements in the throughput of the mobile video traffic and reduction in end-to-end delay for delay sensitive applications. In addition, an assessment of fairness guarantee is considered across all users. Note that this paper considers the impact of the proposed schemes on the transmission/data plane.

Due to spectrum congestion in the commonly used mobile sub-6GHz frequencies, research and measurements in the Millimetre Wave (30-300GHz) bands are required to better understand the medium for 5G and beyond wireless connectivity. In this paper corner diffraction is investigated for an indoor environment in a modern building using a wideband (2GHz) channel sounder at 60GHz. Corner diffraction was measured at five different distances from the corner of interest, with parallel tracks at distances of 0.5m, 1m, 1.5m, 2m and 10m. These measurements were then compared with a Knife Edge Diffraction (KED) model where a `good-fit' was observed. Results showed that for 2m parallel tracks the power fell by 30dB as the user moved just 0.5m into the shadow region. For a 10m parallel track, the same effect was observed after moving 1.2m into the shadow region. Such rapid changes in received power can adversely affect the performance of link adaptation and beam tracking algorithms as well as the efficiency of the higher layer network protocols.

This paper is the first to evaluate the realistic performance of Massive MIMO using real-time channel measurements conducted at 3.51GHz. Our testbed consists of 64 National Instruments Universal Software Radio Peripheral Reconfigurable Input and Output (NI USRP RIO) software defined radios (SDRs) with 128 dipole antennas at the Base Station (BS) side, and 6 USRPs configured as 12 single-antenna User Equipment (UE) nodes. The system operates in a Time Division Duplex (TDD) fashion with a Long Term Evolution (LTE) like Physical Layer framework which, when fully deployed, will enable instantaneous throughput measurements on the Uplink (UL) and Downlink (DL) according to a user-defined frame schedule. In this paper we present real-time UL channel measurements at all BS antenna ports simultaneously, which allows the temporal characteristics of the Massive MIMO channel to be recorded. Eigen-beamforming precoding and a Received Bit-level Mutual Information Rate (RBIR) abstraction simulator are then used to predict the theoretic DL performance. We quantify the co-located Massive MIMO throughput for differing numbers of BS antennas (i.e. 32, 64 and 112 elements) and UE locations for an indoor environment.

In-Band Full Duplex (IBFD) transceivers can potentially provide significant increases in spectral efficiency by allowing simultaneous transmit and receive on the same frequency at the same time. However any such gains are conditional on effective cancellation of the self-interference. The performance of a self-interference cancellation transceiver depends not only on the system design, but also on implementation specific parameters of the transceiver and cancellation circuitry. Thus, realistic evaluation of cancellation performance can only be performed using hardware prototypes. This paper provides a detailed overview of a highly flexible and reconfigurable self-interference cancellation testbed, which combines active cancellation with either separate Tx and Rx antennas, or an Electrical Balance Duplexer, along with digital baseband cancellation. The system can facilitate investigation on self-interference cancellation performance across a range of physical layer configurations, and provide a platform for the rapid prototyping of digital baseband algorithms within an operational self-interference cancelling transceiver.

The current LTE network architecture experiences high level of signalling traffic between control plane entities. As a result many innovative architectures have been proposed including SDN to simplify the network. Cloud RAN on the other hand proposes a pioneering paradigm in terms of sustaining the profitability margin with unprecedented surge of mobile traffic and providing better performance to the end users. Schemes have been proposed to compute and analyse the signalling load in the new SDN LTE architecture; however in our best understanding, none of them consider future CRAN architectures. Thus in this paper we propose a new SDN CRAN architecture and present an analysis of the signalling load, evaluate the performance and compare it against existing architectures in the literature. Evaluation results show significant improvement of the proposed schemes in terms of reduction in the signalling load when addressing several network metrics.

The vast growth in mobile data traffic requires future mobile network infrastructure to support fast overall growing mobile devices while improving the cost and the energy efficiencies. Many recent studies in this context have focused on proposing solutions for end-to-end architectural designs based on flexible allocations of functions, network function virtualization and software-defined implementations (SDN). It's also of upmost importance to introduce a 5G End-to-End architecture that support low latency RAN with joint design of programmable and software's driven networks that can adapt dynamically to the fluctuating traffic demands. This study contributes in this area by providing innovative joint Cloud-RAN-SDN network designs which are expected to reduce operational cost and overall control signaling load. The paper proposes two architectural schemes called C-RAN.C-MME and C-RAN.D-MME. We demonstrate the associated signaling load analysis, evaluate the performance and compare it against existing literature architectures. Evaluation results indicate a substantial improvement in terms of reducing the signaling load taking into account several network metrics. It's worth mentioning that this study shows the impact of the proposed architectures on the signaling plane only, while their benefits for the transmission/data planes is considered in future work.

This paper explores the possibility of opportunistic cognitive radio based offloading for infotainment services in future autonomous vehicles. With the goal of a truly intelligent vehicle, research into the potential of vehicular communications intends to facilitate a wide range of services from safety aspects such as collision prevention to in-car information and entertainment (infotainment) services. With such a broad remit, future vehicles will likely have to rely on multiple communication technologies to successfully service their needs. In this paper we identify the benefits of offloading to WiFi via Drive-thru Internet in vehicular applications but note the damaging handover overheads introduced by such schemes. Instead we propose a method of opportunistic access based on Beacon Interval Whitetime to macro-cellular WiFi access points as a means of sustaining an active connection whilst moving through an urban environment. This will reduce the inefficiencies of successive handovers and enable increased volumes of content to be delivered to the vehicle. We perform an empirical study of the potential for offloading in a representative urban environment and find periods of upto 100 ms per second are available for opportunistic use throughout the test. Additionally we find semi-deterministic opportunities for cognitive access occur following 35% of beacons observed on a WiFi channel.

This paper presents a wideband channel sounder for mmWave radiowave propagation measurements. Three test environments are described and a number of channel sounding validation techniques are proposed. The temporal performance of the sounder is assessed through real-time wideband measurements using two metallic reflectors placed within an anechoic chamber. Furthermore, high directional antennas are employed for the purpose of power validation, as well as for measuring the received signal strength reflected by various building materials. Finally, the depolarization of the building materials is assessed by means of measuring and characterizing the received cross-polar components.

The demand placed on wireless networks operating within fixed allocations of licensed or unlicensed spectrum has introduced the development of cognitive radios, capable of intelligently utilising spare resource whenever it is available. Since WiFi is an ubiquitous method for localized high speed mobile internet access it exists in almost all typical radio environments however the spectrum in which it operates is not exploited to its full potential. This paper introduces a concept called Whitetime to describe the free time between frame transmissions on a typical WiFi channel and across all networks. Initial measurements of real environments indicate that more than 85% of time on a channel is unused however this predominantly consists of short interframe spaces. Detailed analysis reveals periods of inactivity greater than 10ms are synchronised with the beacon interval from observable access points on the channel. For applications that require opportunistic network access such as non deterministic machine to machine (M2M) communication, Whitetime presents a reliable opportunity to transmit packets as long as 1500 bytes using standard physical layer techniques.

In the past few years, extensive research has focused its attention on interference alignment (IA) methods that are aimed at improving the interference networks potential. However, the channel state information (CSI) precision has a correlation with the functionality of IA. When the CSI is completely outdated then there is a possibility that performance effectiveness and gain of IA could be lost. In the current study, a completely outdated source-to-relay CSI within multi-user network is considered followed with the introduction of a collaborative relay system within such a situation to assist in improvement of the networks freedom levels. By examining various outdated CSI situations at transmitter (CSIT), the trade-off involving relay-aided IA (RIA) alongside non-relay-aided IA (NRIA) is obtained. Additionally, the total RIA rate alongside covariance-supported channel estimator is examined.

In this paper, the antenna performance for the 5.2 GHz 802.11n network is compared for four different access points with 3 antenna configurations based on the simulated throughput derived from measurements and a ray-tracing model in a two-storey house. The aim here is not only outline a methodology for access point performance that taking account the user equipment orientation, MIMO data streams that can be supported yielding a total throughput metric, but also ascertain the accuracy of the ray tracing model. In order to demonstrate this, two commercial access points are compared with two alternative antenna configurations: an orthogonal array of dipoles and a linear array of vertical dipoles. Radiation patterns, directivity and polarization mix are presented for all access points. Physical layer throughput is computed for all modulation and coding based on the measured spectral responses and simulated channel matrix. Results show the orthogonal dipole array has the best overall performance with the average throughput of 137 Mbps while vertical dipole array works worst with 106 Mbps. The ray tracing model produce results within 8% of differences and more importantly provide the same performance ranking order.

Spectrum shortage in the conventional microwave bands coupled with the need for multi-gigabit per- cell capacities has motivated the use of the mmWave bands in future 5G networks. In this paper 3D ray tracing is used to derive key statistical parameters for the mmWave channel. These include path loss, LoS probability, K-factor, RMS delay spread and angular spreads. The mmWave parameters are directly compared to the microwave channel present in LTE-A. Cellular deployment in both bands is then simulated for a 500 m by 500 m regqasion of central Bristol. The impact of different antenna geometries is also studied. Due to the trade-offs between the number of required BSs and the cost of network deployment, we propose a converged mmWave/microwave network for improved user experience.

This paper presents a path-loss prediction analysis and a performance evaluation of the 60GHz IEEE 802.11ad standard. Received signal strength measurements were performed in indoor locations and the decay coefficients were estimated over a number of measured routes. Furthermore, real world maps were incorporated into a 3-D mm wave ray- tracing tool for the purpose of employing realistic channel matrices into an 802.11ad bit level simulator. Finally, performance metrics for various MCS modes are presented and analyzed for both Orthogonal Frequency Division Multiplexing (OFDM) and Single Carrier (SC) transmissions.

This paper presents a practical demonstration of the benefits of limited feedback beamforming for mmWave applications. Different beamforming strategies are presented, analysed and compared. Compressive sensing (CS) is investigated to accurately estimate the sparse Power Angular Profile (PAP) of the mmWave channel. This information is then used to estimate the antenna array weights required to steer the beam to maximise received SNR. This scheme is particularly attractive for outdoor mmWave networks where large antenna arrays are necessary to mitigate high observed pathloss. Codebook based beamforming strategies are also analysed. Using a practical mmWave antenna away, measurements are reported for a controlled multipath channel that compare the performance of the proposed beamforming strategies. In terms of received SNR CS based beamforming was shown to outperform more traditional codebook based solutions by to 5dB.

This paper introduces a novel 128-element sub-6 GHz Massive MIMO (Multiple-Input Multiple-Output) testbed being developed by the University of Bristol in collaboration with National Instruments and Lund University as part of the BIO (Bristol is Open) city testbed. Distributed Massive MIMO systems avoid large single-piece antenna facets and their associated windage problems and also offer greater spatial diversity, but node synchronization and phase alignment becomes more challenging. Simulated network performance of a co-located 128-element 3D Multi-User MIMO system at a possible test site in Bristol are presented based on our 3D channel propagation model.

Electrical balance duplexers can provide high transmit-to-receive isolation whilst facilitating transmission and reception from a single antenna, can be implemented on-chip, and are widely tunable, making this an attractive technology for implementing full duplex architectures in small form factor devices. This paper presents measurements from a novel hardware prototype full-duplex transceiver architecture combining electrical balance and active analog cancellation. The prototype duplexer achieves >80dB transmit-to-receive isolation over a 20MHz bandwidth at both 890MHz and 1890MHz, exceeding the performance of antenna separation architectures where the antenna isolation is limited to the levels achievable in hand held devices.

The non-orthogonal multiple access (NOMA) is regarded as a promising multiple access technique for 5G network. In this research, comparing with the joint transmission (JT) NOMA in Coordinated Multi-Point (CoMP) system, we propose a novel opportunistic NOMA (ONOMA) scheme, which aims to improve the efficiency of the successive interference cancellation (SIC) decoding in conventional NOMA. Besides, by comparing with the JT-NOMA in CoMP system, the data rate of the ONOMA is shown in the numerical results under the ideal and non-ideal scenarios.

A key issue impacting the transmit to receive (TX-RX) isolation in Electrical Balance (EB) Duplexers is dynamic matching of the antenna impedance by the balancing network. Circuit simulations including embedded antenna measurements have been used to investigate the variation in TX-RX isolation due to interaction with the user and the local environment. Simulated results at 847MHz and 1960MHz show user interaction causes over 25dB of variation in the mean isolation over a 20MHz bandwidth. However, in reflective indoor environments, user interaction on average improves TX-RX isolation. This observed performance improvement is a result of reduced antenna radiation efficiency, which makes reflections from objects at longer ranges less significant and thereby reduces frequency domain variation in the antenna reflection coefficient.

This paper investigates electrical balance (EB) in hybrid junctions as a method of achieving transmitter-receiver isolation in single antenna full duplex wireless systems. A novel technique for maximizing isolation in EB duplexers is presented, and we show that the maximum achievable isolation is proportional to the variance of the antenna reflection coefficient with respect to frequency. Consequently, antenna characteristics can have a significant detrimental impact on the isolation bandwidth. Simulations that include embedded antenna measurements show a mean isolation of 62 dB over a 20-MHz bandwidth at 1.9 GHz but relatively poor performance at wider bandwidths. Furthermore, the operational environment can have a significant impact on isolation performance. We present a novel method of characterizing radio reflections being returned to a single antenna. Results show as little as 39 dB of attenuation in the radio echo for a highly reflective indoor environment at 1.9 GHz and that the mean isolation of an EB duplexer is reduced by 7 dB in this environment. A full duplex architecture exploiting EB is proposed.

The demand for wireless applications with high data rates and strict quality of service (QoS) requirements continues to grow. This has placed huge obligations on wireless service providers to meet this need when rolling out radio access networks (RAN) in order to meet these capacity and coverage requirements. A distributed antenna system (DAS) is a RAN architecture that improves both coverage and capacity for both `dead-spots' and `hotspots'. In this paper, we investigate the energy efficiency gains of DAS in the case of non uniform user distributions. This investigation is conducted for outdoor hotspot scenarios and results show that the energy reduction gain (ERG) of DAS when Remote Radio Heads (RRHs) are located coincident with the hotspots improves with increasing numbers of users per hotspot as well as with increasing transmit power of the RRH. When RRHs are deployed coincident with hotspots, energy savings of 20% can be achieved. This is in contrast to the case when they are not, where losses of energy efficiency can be observed to result from the use of DAS.

In this paper, for the first time real--world channel measurements are used to analyse the performance of spatial modulation (SM), where a full analysis of the average bit error rate performance (ABER) of SM using measured urban correlated and uncorrelated Rayleigh fading channels is provided. The channel measurements are taken from an outdoor urban multiple input multiple output (MIMO) measurement campaign. Moreover, ABER performance results using simulated Rayleigh fading channels are provided and compared with a derived analytical bound for the ABER of SM, and the ABER results for SM using the measured urban channels. The ABER results using the measured urban channels validate the derived analytical bound and the ABER results using the simulated channels. Finally, the ABER of SM is compared with the performance of spatial multiplexing (SMX) using the measured urban channels for small and large scale MIMO. It is shown that SM offers nearly the same or a slightly better performance than SMX for small scale MIMO. However, SM offers large reduction in ABER for large scale MIMO.

In this paper we analyze measurements of short range ultra-wideband (UWB) multiple input multiple output (MIMO) transmission. 2 × 2 near field UWB measurements were undertaken in an anechoic chamber using a Medav UWB MIMO channel sounder. Channel characteristics were recorded for transmitter to receiver separations of 0.1 m to 0.7 m. Two independent channels were achieved, and channel capacity is twice as high as capacity of a single channel.

This paper presents a wireless multiple-input/multiple-output (MIMO) testbed for the UK-China Science Bridges Project: R&D on Beyond Fourth Generation (B4G) Wireless Mobile Communications (UC4G). An introductory review of other approaches is first presented and then subsequently the motivation, architecture and specifications of the UC4G testbed are outlined. The flexibility of operation, versatility of function and modular design of the UC4G testbed are shown to be its key benefits.

This paper proposes a non-stationary wideband geometry-based stochastic model (GBSM) for multiple-input multiple-output (MIMO) high-speed train (HST) channels. The proposed model has the ability to investigate the non-stationarity of HST environment caused by the high speed movement of the receiver. Based on the proposed model, the space-time-frequency (STF) correlation function (CF) and STF local scattering function (LSF) are derived for different taps. Numerical results show the non-stationarity of the proposed channel model.

An ultra-wideband (UWB) antenna array is designed to satisfy given constraints in a (large) contiguous frequency range. Space limitations are tight in many applications and it is often infeasible to design the antenna array with preferred element spacings of λ/2 at the centre frequency. In this study, the authors compare uniform linear UWB antenna arrays and non-uniform reduced-aperture UWB antenna arrays with more compact element spacings following a geometric progression. The authors analyse and discuss performance degradations with respect to irregularly spaced monopoles. The manufactured non-uniform linear UWB antenna arrays are characterised by measurements in an anechoic chamber. The results are correlated with the arrayfactor and compared with measured data of uniform linearly spaced monopoles in the time and frequency domains. It is found that a compact non-uniform linear UWB array with element spacings following a geometric progression can be designed such that its main lobe performs very close to that of a uniform linear array, with an antenna array size reduction of 23.6% for a seven-element non-uniform linear UWB array.

The selection of optimum frequency channels by cognitive radio devices is considered challenging, particularly when such devices have to mutually co-exist and also take account of existing users of the spectrum. In addition to the problem of accurate spectrum sensing, there are additional considerations that result because of the dynamic capabilities of the terminals. Particular problems include that of allowing terminals to discover each other when they may be tuned to different frequencies, operating without a common control channel. This work has developed a spectrum access algorithm by which cognitive radio terminals monitor the available spectrum, discover each others’ presence and collaborate to select the optimum communications channel. This is achieved while avoiding interference to incumbent users. In contrast to previous work in this area, the proposed algorithm is of low complexity, requiring the terminals neither to dedicate part of their operational time to spectrum sensing, nor to periodically switch to a control channel. It is thus suitable for implementation in simple terminals. A simulation tool, which models the interaction and behaviour of the terminals, has been produced and used to test the performance of the algorithm in a demanding environment. It was shown that the algorithm permits a high level of data throughput, with protection for incumbent spectrum users.

This study investigates two areas relating to practical multi-antenna ultra-wideband (UWB) indoor channel: the frequency dependency in line-of-sight (LOS) and non-LOS (NLOS) environments, and the differences in spatial characteristics when using virtual and physical arrays in multi-antenna channel characterisation. The authors quantify the frequency dependency of the channels in terms of their resilience to spatial fading and system capacity. The composite radiation pattern of the antenna and channel will impact the channel properties, the authors investigate these changes by measurement and analysis of channel data taken using both physical and virtual array configurations at the same locations. They find that at lower frequencies the resilience to spatial fading in LOS channels was less than half of the theoretic maximum. The difference in capacity and diversity gain between the LOS and NLOS channels was found to decrease with centre frequency. The different rates of increase result in a 4% average increase in capacity for a 500-MHz subband centred at 4.5 and 9.5 GHz for the LOS scenarios, whereas an increase of only 0.7% was experienced by the NLOS scenarios. In addition, this analysis indicates that the estimated diversity and capacity of multi-antenna channels decrease when using physical antenna arrays for measurements.

This paper presents results from a novel OFDMA multi-cell mobile broadband system-level simulator. The tool is used to statistically characterize uplink and downlink inter-cell interference. Fully loaded interference studies cannot be performed on real-world networks until they have been fully deployed. As such, interference analysis and management must be accurately performed pre- deployment using detailed network simulators. This paper discusses our simulator architecture and the steps necessary to reduce computational complexity inherent in such simulations. The paper then demonstrates the impact of inter-cell interference in OFDMA networks. The results demonstrate that it is the frame-to-frame fluctuations in interference, and not the received signal level, that dominate inaccuracy in the Channel Quality Index (CQI). CQI errors lead to the wrong MCS mode being chosen by the AMC algorithm and this leads to throughput reduction. Results show that this problem is exacerbated by high vehicular speeds and the absence of interference randomization. Compared to ideal MCS selection, throughput reductions of up to 45% are reported.

This paper presents results from a novel OFDMA multi-cell mobile broadband system level simulator. The tool is used to statistically characterize uplink and downlink inter-cell interference. Without suitable interference management, multi-billion dollar networks can collapse under the strain of heavy traffic loads. Fully loaded interference studies cannot be performed on the network until it has been fully deployed. As such, interference analysis and management must be accurately performed pre-deployment using detailed network simulators. System level simulation is a highly computationally intensive procedure. This paper discusses the simulator architecture (and steps taken to reduce computational complexity) and then demonstrates the impact of inter-cell interference in OFDMA networks. In an interference limited scenario the results demonstrate that it is the frame-to-frame fluctuations in interference, and not the received signal level, that dominate inaccuracies in the Channel Quality Index (CQI) prediction. CQI is used in the fast link-adaptation process to select the MCS mode for each user on a frame-by-frame basis. Results show that incorrect Modulation and Coding Scheme (MCS) modes are chosen up to 40% of the time when the CQI is delayed by 3 frames and the user is interference limited. Perfect MCS selection is then shown to improve user throughput by up to 50%.

This paper exploits the use of location information of wireless terminals to improve the performance of a beamforming system and support multiple access. Based on a system providing real-time 3D coordinate information of all terminals in the environment, a novel feedback-free multi-user scheduler is proposed. It chooses a set of angularly separated wireless terminals for simultaneous transmission in order to reduce the impact of mutual interference on overall system performance. If additional feedback of signal-to-interference and noise ratio is available, a location-assisted multi-user scheduler is also suggested to further improve the overall system performance. Both multi-user schedulers are based on a rate-greedy approach. To improve the fairness in resource allocation without introducing additional feedback overhead, a modified proportional fair algorithm (PFA) based on location information can be applied to a beamforming OFDMA system. By scaling the window length of PFA, a range of trade-offs between fairness and rate can be achieved.

This paper proposes a new wireless scheduling methodology which takes advantage of future predictions of data rate for the several users competing for physical access. Based on a system which allows low-cost 3D mapping of an environment in real-time, the predictions are obtained via either low-resolution path-loss prediction given the physical structure of the surroundings or by reference to data rates achieved by previous visitors to a locality. The proportional fair scheduling (PFS) metric is extended to include measures of the future rates users may achieve, leading to a new family of schedulers. They show useful fairness improvements over PFS in exchange for a small capacity loss, and allow a number of configuration options and a range of trade-offs between fairness and capacity.

This paper investigates OFDMA downlink resource (spectrum and power) sharing algorithms for fixed relay stations. Iterative waterfilling is used in the power allocation process and shown to result in an optimal power allocation solution for multiple-relay networks. Iterative waterfilling is then developed with a power reallocation process to achieve more efficient spectrum utilization. Furthermore, joint subcarrier and power allocation algorithms are proposed with and without consideration for multiuser fairness. Simulation results show that iterative waterfilling can significantly improve the system capacity. The proposed joint allocation algorithm achieves gains of up to 21% in system capacity compared to iterative waterfilling. The joint allocation algorithm with fairness is shown to degrade system capacity, however fairness is important in a multiuser network.

The end-to-end performance of applications in broadband wireless networks is a key concern from the user perspective. Mobile WiMAX is designed to support a wide range of different applications including media streaming, voice-over- IP (VoIP), video conferencing, and web browsing. All of these applications require different levels of quality of service (QoS) and this imposes a variety of different performance requirements on the WiMAX physical (PHY) and medium access control (MAC) layers. For example, TCP-based applications such as FTP or Web-browsing use automatic-repeat-request (ARQ) to tolerate a relatively high PHY packet error rate (PER), whilst a UDP-based application such as video streaming requires very low PER due to the lack of ARQ. This means that the same WiMAX system configuration needs to be able to provide different performance levels for different types of applications in order to operate efficiently. This paper analyses and compares the performance of a multi-input multi-output (MIMO) mobile WiMAX system in terms of achievable throughput and operating range for different PHY PER requirements. The paper shows that TCP applications can achieve higher throughputs and operating ranges due to higher PER tolerance. It also shows that this important QoS parameter affects the AMC switching points. Cross-layer interaction is required between the PHY and the scheduler in the MAC layer in order to efficiently fulfil the various QoS requirements at the application layer.

In this paper, algorithms on distributed resource (spectrum and power) sharing for relay stations are investigated for downlink transmissions in an OFDMA-based relay-aided cell. Both system capacity and user fairness are considered. By grouping the relay stations into coalitions according to the set of users they are relaying, the optimal resource allocation can be solved by considering resource allocation within and among the coalitions. The algorithm for intra-coalition resource allocation is proposed by utilizing the key observation: for each data symbol transmitted from the base station to a user (in a subcarrier), only one among all the available relay stations is required to relay the symbol. The inter-coalition resource allocation is modeled by both a non-cooperative and a cooperative game, where the cooperative game is solved by a nonsymmetric Nash bargaining solution. Simulation results show that the non-cooperative algorithm outperforms random allocation by approximately 50% in system capacity with 3 relay stations in each coalition. The cooperative algorithm has approximately 5% loss in system capacity comparing with the non-cooperative algorithm, but achieves a significant gain in terms of fairness performance.

In this paper, we focus on the design of efficient resource allocation algorithms for a multihop cellular network, with orthogonal frequency division multiple access (OFDMA) as downlink transmission technique and utilizing decode-and- forward cooperation strategy. Our objective is to maximize the total capacity based on the constraint of individual transmission power at each transmitter. We also consider users' QoS requirements by maximizing the total capacity while giving users proportional fairness weights according to their data requirements. We show that for each data symbol (at each subcarrier) transmitted by base station, the best relaying strategy is to let only one among all the relay stations perform the relaying task. This is true for both objectives of maximizing total capacity and maximizing capacity with proportional fairness constraint. We then propose efficient greedy algorithms for both centralized and distributed resource allocations based on our analysis. Simulation results indicate that our proposed algorithms effectively enhance the total capacity.

Precise measurement and modelling of the Direction of Arrival (DoA) and Direction of Departure (DoD) in the Multiple-Input Multiple-Output (MIMO) channel based on their correlation properties and joint distributions have not been investigated properly. Our main contribution in this paper is two fold. Firstly, based on a modified Gumbel's bivariate exponential approach, we propose a generic indoor MIMO channel modelling framework particularly for the double-directional 'general trend'' that has been observed in these dual-spatial domains. Secondly, we propose a transformation technique to generate the pair wise DoA and DoD components incorporating the correlation properties of the variates around the full azimuth range. Results show that the newly proposed modelling approaches provide a good match to the real data, offering an alternative hassle-free method to generate realistic directional-based stochastic MIMO channel responses in different indoor environments.

Recently, there has been an explosion of growth in research on MIMO systems, but little has been published characterising performance in realistic environments. This paper quantifies the performance of MIMO WLANs in outdoor environments, and compares performance between spatial multiplexing and space time block coding processing approaches. Packet Error Rate (PER) and throughput performance results are presented under different channel conditions. A WLAN physical layer simulator employing MIMO techniques and a propagation modelling tool are combined in order to evaluate the coverage and throughput enhancements of WLANs for the 2×2 and 4×4 MIMO cases.

The directional dispersion of multipath energy arriving at the receiver is an important quantity, as increasing it leads to lower correlation between spatial diversity elements. The `RMS angular spread' metric, commonly used to measure this, is only applicable for relatively small angles in the 2-D plane. The quantification of the 3-D directional spread is not straighforward, and must be justified analytically by showing its relation to spatial selectivity, and hence spatial correlation. In this paper, an analysis of the second order moments of spatial fading has been used to define a spatial covariance matrix, which fully describes the second order moments of the directional energy distribution. An eigenvalue decomposition of this matrix has been used to propose a concise quantification of directional spread that is applicable for arbitrary distribution of multipath rays or clusters in 3-D directions. The effect of array orientation has also been investigated.

This paper describes an outdoor MIMO measurement campaign conducted in the 2 GHz band employing a sounding bandwidth of 20 MHz. The study aimed to compare the MIMO performance of two prototype devices with a reference dipole antenna module. The results obtained reveal a systematic blocking of one of the PDA antenna ports with the user's thumb alongside a significant reduction in the available MIMO channel capacity. The laptop MIMO enabled device was found to offer good MIMO capacity enhancement, matching the performance achieved with the dipole antennas

A novel smart antenna is described, which is conceived as a low complexity inexpensive upgrade for IEEE 802.11b/g wireless local area network (WLAN) systems. Using smart antennas to increase network throughput by improved spatial reuse is not normally possible for systems using the 802.11 medium access control (MAC), unless centralised channel access control is enforced. However, it is shown through simulation that the proposed antenna can have sufficient beam and null steering capabilities to allow two 802.11g transmissions to co-exist in close proximity. This is achieved with the standard distributed channel access algorithms and therefore has potential to increase system capacity. Accuracy in simulation is ensured through a combination of MAC functionality modelling using OPNET and 3 dimensional (3D) propagation modelling using ray tracing.

In this paper, the results of an on-body, line-of-sight (LOS), multiple-input, multiple-output (MIMO) channel sounding investigation are reported. Transmission takes place between on-body antennas and a uniform linear array, positioned at close range and within line-of-sight of each other. Due to the on-body location of the antennas, the influence of actual radio channel can no longer be separated from the effects of user behaviour and body proximity. This paper therefore argues that the composite channel must be considered. Post-processing of the recorded channels shows that their capacities outperform those from equivalent Rayleigh channels. Taking into account the clear LOS in the experiment, this appears counter-intuitive to the well-known fact that uncorrelated scattering results in high capacity. An analysis of the results highlights some less familiar facts and leads to an overview of capacity generating mechanisms in MIMO systems. The paper adds an analysis of the sensitivity of the capacity to the properties of the channel matrix, which is used for a discussion of the practical use of MIMO in personal and body area networks

Precise modelling of the direction of arrival (DoA) and direction of departure (DoD) of multipath components (MFCs) in a multiple-input multiple-output (MIMO) channel based on correlation properties and joint distributions have not been investigated properly. This paper examines the correlation between DoAs and DoDs, and presents a modelling approach particularly for the "general trend" of these dual-spatial domains, based on statistical results obtained from several indoor MIMO measurement campaigns. We show that the Laplacian distribution provides better fits for the directional parameters than the normal and Von Mises distributions. The "general trend" of DoA/DoD joint distribution and power density are modelled using a modified Gumbel's bivariate exponential approach. Results show that the proposed bivariate distribution provides a good match to the real data, and offers an alternative approach to generate realistic directional-based stochastic MIMO channel responses in different environments

This paper examines how the capacity of three multiuser multiple-input multiple-output (MIMO) algorithms is affected by a delay in computing and feeding back the transmit (and receive) weighting matrices used by the algorithms. Iterative schemes determining transmit weights that achieve the system-wide Nash equilibrium (NE) or block-diagonalization (BD) of the overall system channel matrix are compared as is a scheme applying successive diagonalization (SD) among users. The NE is found to be far the most robust to out-of-date channel information, whilst SD is found to suffer badly at even the shortest delay considered here (about 0.9 ms); BD's losses vary over a wide range depending on the delay. For the two iterative algorithms we also examine the tradeoff between better convergence and less delay. In both cases, the loss in capacity for weaker convergence is very small, and so it is desirable to minimize the delay and thus the capacity loss caused by it

A new pre-FFT synchronization method for OFDM is proposed and assessed that gives improved performance in multipath channels. The technique can be used for a range of OFDM signal parameters, and channel environments. The relative increase in complexity over existing correlation based methods is less than 10%

Single-carrier (SC) wireless communication systems generally require knowledge of the channel and the variance of the additive noise process to equalize a received message. Obtaining this information can be straightforward in stationary environments; however, these parameters constantly change in mobile environments. In this paper, we propose novel algorithms for estimating and tracking the channel and noise variance in SC systems by exploiting a unique word (UW) extension. These UW-based algorithms benefit from low complexity and lend themselves to SC systems employing frequency-domain equalization at the receiver

A novel stochastic wideband dynamic spatio-temporal indoor channel model which incorporates both the spatial and temporal domain properties as well as the dynamic evolution of paths when the mobile moves is proposed based on the concept of a Markov process. The derived model is based on dynamic measurement data collected at a carrier frequency of 5.2 GHz in typical indoor environments. Multipath components are estimated using the super-resolution frequency domain space-alternating generalized expectation maximization algorithm prior to identification of path "birth" and "death" using a new data analysis method. Analysis shows that multiple births and deaths are possible at any instant of time. Furthermore, correlation exists between the number of births and deaths. Thus, an M-step 4-state Markov channel model (MCM) is proposed in order to account for these two effects. The spatio-temporal variations of paths within their lifespans are taken into consideration by the spatio-temporal vector which was found to be well modeled by a Gaussian probability density function while the power variation can be modeled by a simple low-pass filter. In addition, the methodology used to extract the MCM parameters from the measurement data is also presented. Due to the distinction in the birth-death statistics, the model is generalized through segmentation of the measurement runs and can be completely parameterized by several sets of Markov parameters associated with the type of environment and scenario under consideration. The implementation of the model is also detailed and, finally, the model is evaluated by comparing key statistics of the simulation results with the measurement results.

Channel estimation and tracking pose real problems in wideband single-carrier wireless communication systems employing multiple transmit and receive antennas. An alternative to estimating the channel is to adaptively equalize the received symbols. In this paper, we present an adaptive equalization algorithm for implementation in multiple-input multiple-output (MIMO) single-carrier (SC) systems with frequency-domain equalization (FDE). Furthermore, we outline a novel method of reducing the overhead required to train the adaptive equalizer. Other computationally efficient adaptive MIMO SC-FDE algorithms can only be applied to space-time block-coded (STBC) architectures. The algorithm detailed in this paper can be implemented in STBC systems as well as in broadband spatial multiplexing systems, making it suitable for use in high data rate MIMO applications.

This work presents a MIMO throughput performance analysis of dynamic wideband double-directional channel measurements that were recently obtained by the University of Bristol. Identical 16-element uniform circular arrays (UCAs) were employed at both ends of the link and the parameters of the multipath components (MFCs) were extracted. In this paper, the performance analyses of several 4/spl times/4 subarrays of the 16/spl times/16 measurement arrays are presented. The MIMO response of these channels was synthesised from the extracted MFCs. A comparison is then made between the capacity estimates from the directly measured and synthesised MIMO channels. This was found to show good agreement.

We investigate a new class of training sequences that are optimal for least squares (LS) channel estimation in systems employing transmit diversity and single-carrier (SC) modulation with a cyclic prefix (CP) extension. The sequences have a constant envelope in the time domain and are orthogonal in the frequency domain. Transmission of these sequences facilitates optimal (in the LS sense) estimation of the channel impulse response at the receiver while precluding the peak-to-average power ratio problem that is inherent in other CP-based architectures such as orthogonal frequency division multiplexing.

Recent developments in orthogonal frequency division multiplexing (OFDM) and single-carrier frequency-domain equalization (SCFDE) have sparked debate about the superiority of one method over the other. In this paper, we further this debate by comparing the theoretical performance of OFDM and SCFDE when each is implemented in one of two different multiple-input multiple-output (MIMO) architectures: spatial multiplexing and space-time block codes. This study focuses on the use of MIMO-OFDM and MIMO-SCFDE in wireless local area network (WLAN) applications. Performance is given in terms of the packet error rate (PER) and the throughput of the systems.

A high efficiency and linearity PA solution is presented based on hybrid linearisation schemes that employ complementary techniques to overcome the weakness (or constraints) of either method on its own. In particular, the high linearity of Cartesian loop is combined with the high efficiency of envelope elimination and restoration (EER). The results of a prototype hybrid linearised PA built using digital generation of the phase and amplitude signals for the EER amplifier, and utilizing a digital Cartesian feedback loop are presented. Limitations of the amplitude modulator on system performance are demonstrated using PWM and delta-modulated switching amplifiers.

In this paper, a new statistical wideband indoor channel model which incorporates both the clustering of multipath components (MPCs) and the correlation between the spatial and temporal domains is proposed. The model is derived based on measurement data collected at a carrier frequency of 5.2 GHz in three different indoor scenarios and is suitable for performance analysis of HIPERLAN/2 and IEEE 802.11a systems that employ smart antenna architectures. MPC parameters are estimated using the super-resolution frequency domain space-alternating generalized expectation maximization (FD-SAGE) algorithm and clusters are identified in the spatio-temporal domain by a nonparametric density estimation procedure. The description of the clustering observed within the channel relies on two classes of parameters, namely, intercluster and intracluster parameters which characterize the cluster and MPC, respectively. All parameters are described by a set of empirical probability density functions (pdfs) derived from the measured data. The correlation properties are incorporated in two joint pdfs for cluster and MPC positions, respectively. The clustering effect also gives rise to two classes of channel power density spectra (PDS)-intercluster and intracluster PDS-which are shown to exhibit exponential and Laplacian functions in the delay and angular domains, respectively. Finally, the model validity is confirmed by comparison with two existing models reported in the literature.

In this paper, the serial interference cancellation (SIC) technique and the frequency domain SAGE (FD-SAGE) algorithm are jointly used to detect and estimate the radio channel parameters of interest. The implementation of the SAGE algorithm in the frequency domain is novel. Furthermore, the parallel interference cancellation (PIC) technique in the standard SAGE algorithm is replaced by the SIC technique. The SIC technique demonstrates more stable performance especially in a multipath rich environment. The two-dimensional (2-D) FD-SAGE algorithm and the SIC technique are introduced and their performance demonstrated by using real indoor channel measurement data to jointly estimate the number of multipath components (MPCs), their time-of-arrivals (TOAs), angle-of-arrivals (AOAs) and complex amplitudes. Their performance is evaluated and compared using synthetic data and results using 2-D unitary ESPRIT (another form of super-resolution algorithm).

The exponential growth of cellular radio, WLANs and the Internet sets the context for a discussion on the, role and objectives of 4G. In this paper OFDM is proposed as a leading candidate for a 4G cellular communications standard. The key design considerations and link parameters for a 4G OFDM system are identified and initial physical layer performance results are presented for a number of transmission modes and channel scenarios. Additionally, space-time techniques are considered as a means of enhancing the performance of the coded OFDM system.

A novel approach in quantifying code orthogonality factor for UTRA-FDD systems is extended in this paper to include the impact of down-link beamforming. The effects of fixed beamforming on code orthogonality is analysed through the aid of real channel data taken from urban cellular environments. The results show a significant overall improvement in code orthogonality with beamforming, especially in the case of an urban large cell.

The paper presents the analysis of spatial correlation in MIMO channels, calculated from data measured in office environments at 5.2 GHz. Results are compared with those from channels generated using a stochastic MIMO channel model and the effect of different comparison metrics is shown. The suitability of the stochastic model under different propagation conditions is also investigated. The performance of the models has been shown to be good under non-line-of-sight (NLOS) propagation, although one model was shown to fail under conditions where the correlation amongst the elements of one array was not independent of the antenna element at the other array. It is shown how situations in which array correlation varies with array element can occur in line-of-sight (LOS) conditions, and how under these extreme circumstances, both stochastic models are unsuitable.

Herein, results of 5.2 GHz wideband indoor multiple input multiple output (MIMO) channel measurements under the EU IST SATURN project are reported. Our investigation shows that for non-line-of-sight (NLOS) cases, the average power delay profiles fit the exponentially decaying curve quite well, therefore a simple wideband model for single-input single-output (SISO) proposed in COST259 has been used in our model. Furthermore, the investigations show that the MIMO channel covariance matrix of each normalized tap of the impulse response could be well approximated by the Kronecker product of the covariance matrices seen from the transmitter and receiver respectively. Based on the above results, a wideband statistical model is presented. Monte-Carlo simulations show reasonably good agreement between the measured data and our model. Finally, we use this model to show some capacity characteristics of Hiper-LAN/2 channels in NLOS indoor scenarios.

Downlink beamforming in a WLAN-type system employing access points and mobiles equipped with multiple antennas and associated receivers and transmitters are considered. The beamforming aims at maximizing the performance under constraints on transmit power or equivalent isotropic radiated power (EIRP). Solutions for the two constraints are derived and investigated using simulated and measured channels. Our simulation and experimental results shows that performance gains of 8-10 dB when using four directional transmitter antennas and two receive antennas (as compared with a base-line one-transmit two-receive), are possible under both constraints. For simulated channels, a delay between channel estimation and use of the same channel of up to 10% of the (inverse of the) Doppler frequency only degrades performance some tenths of a dB. In our measurements, very small degradations are seen with delays of up to 130 ms. The measurements were made under relatively stationary conditions with only occasionally moving people. Two different strategies for updating the beamforming vectors: sounding and ping-pong, are also considered in the paper.

Herein, results from measurements conducted by the University of Bristol are presented. The channel characteristics of multiple input multiple output (MIMO) indoor systems at 5.2 GHz are studied. Our investigation shows that the envelope of the channel for non-line-of-sight (NLOS) indoor situations are approximately Rayleigh distributed and consequently we focus on a statistical description of the first and second order moments of the narrowband MIMO channel. Furthermore, it is shown that for NLOS indoor scenarios, the MIMO channel covariance matrix can be well approximated by a Kronecker product of the covariance matrices describing the correlation at the transmitter and receiver side respectively. A statistical narrowband model for the NLOS indoor MIMO channel based on this covariance structure is presented.

This paper addresses a number of key issues that affect the modelling of propagation in mobile-to-mobile communications. Current examples of such systems include intelligent relaying, Bluetooth and other ad-hoc solutions. The key difference in mobile-to-mobile communications is that both transmit and receive antennas are likely to be closer to the ground than those in conventional cellular networks. In addition, they may also be located in less favourable propagation positions, such as in users' pockets, bags or desk drawers. Two measurement programmes have been performed to investigate the propagation issues relating to intelligent relaying in the 2 GHz band. The first campaign concerns path loss measurements in an indoor environment where the heights of the antennas are lower than those in conventional cellular networks. The second set of data was taken to determine variations in the terminal radiation pattern given the close proximity of the human body. A number of single ray illumination measurements were performed inside an anechoic chamber. The results indicate that for the environments under consideration, an additional attenuation factor should be added to the path loss model to compensate for the unfavourable location of the terminals in a mobile-to-mobile propagation environment. This attenuation is dependent on the heights of the antennas involved, and is in the range 0-8 dB.

This paper identifies the issues that are important in the design of a SDR receiver. Receiver architectures are first discussed, and the conclusion drawn that the conventional superheterodyne structure is most appropriate for a SDR receiver. Issues associated with image rejection, and receiver linearity are discussed. The design of a sweepable preselect filter is discussed in detail. Design considerations for a practical SDR test-bed are presented.

The measurement and characterisation of multiple-input multiple-output (MIMO) channels has gained increasing attention over recent years. Previous analysis of MIMO measurements has generally focussed on the evaluation of the capacity of such systems in differing locations and configurations. We present measurements made in an indoor environment at 5.2 GHz, specifically to investigate temporal channel variation. Line-of-sight and non-line-of-sight situations are compared by means of an analysis of the rate of variation in the powers of the 'orthogonal spatial channels' comprising the overall MIMO channel. The conclusions are extended to encompass the rate of variation in performance of MIMO channels under different environmental conditions.

Given the potential market for wireless LAN (WLAN) technology, capacity within the available 2 and 5 GHz frequency bands is likely to fall short of market demands. Multiple-sector antenna arrays are known to be able to provide capacity enhancement by means of interference reduction through spatial filtering. In this paper, the performance of an eight-element circular array acting as an access point (AP) in indoor wireless LAN environments is analysed. Spatial-temporal channel data used within the analysis was obtained from physical measurements within numerous indoor environments using a state-of-the-art wideband vector signal analyser. Performance analysis of the array employing a switched-sector controller within Hiperlan/2 standard is appraised in terms of carrier-to-interference (C/I) ratios. Results show that an isolation of up to 15 dB can be achieved between the two polarisations by means of using an antenna array at the AP. It is shown that 54% of the time, 2 users per timeslot can be supported by the system.

The benefits of polarisation diversity as a fade countermeasure within second generation cellular networks are well known. Capacity enhancement by means of joint detection is also a well-regarded technique for interference-limited systems such as TD-CDMA (time division-code division multiple access) within the UMTS (Universal Mobile Telecommunication System) wireless standards. However, relatively little attention has been focussed towards the potential gains of combining these two technologies. This paper considers, by means of a series of wideband dual polar channel measurements from a field trials campaign taken within the lower UMTS band (1910-1915 MHz), the relative merits of this approach when applied to a multistage interference canceller. This paper shows that signal strength improvements of between 2.5 dB and 3.2 dB at the 1% cumulative level were observed by applying dual polarised antennas at the receiver.

A WLAN-type scenario where a base-station (access point) equipped with multiple antennas is transmitting and a mobile with only a single antenna is receiving, is considered. Three approaches are investigated: grid-of-beams (GOB), maximal-ratio (MR) and equal-gain (EG), each with one of the following constraints 1) total transmit power over all antenna elements, 2) maximum power on any antenna element 3) equivalent isotropic radiated power (EIRP). The median diversity gain for the MR approach is estimated to be 14.8 dB, 10.6 dB, and 11.4 dB using constraint 1-3, respectively at 5.2 GHz in a modern university building at 5-50 meter range. For GOB corresponding numbers are 9.4 dB, 9.4 dB, 2.3 dB and for EG 13.9 dB, 13.9 dB, 10.3 dB, respectively. These results are virtually independent of delay between channel estimation and use of the same estimate, when the delay is less than 130 ms. This result is obtained although there were people moving in the environment. The diversity gain under the EIRP constraint is encouraging and shows that coverage improvements are possible even under EIRP limitations.

To date, research into reconfigurable mobile communications has predominantly focussed on the software radio concept, and specifically on the hardware technologies required to move physical layer processing into a programmable environment. Although an interesting and necessary challenge, this only represents a fraction of the overall support and technology required to realise the potential of the concept. Other necessary developments include network/terminal cooperation for seamless inter-standard handoff, QoS management, a secure software download mechanism, terminal software architecture supporting reconfiguration, configuration management, capability negotiation. Summarising results from early IST-TRUST (Transparently Reconfigurable Ubiquitous terminal) project deliverables, this paper describes the likely overall system environment, and the key technical challenges to be researched in TRUST for realising a reconfigurable terminal to meet the needs of users within that environment.

This paper presents the results of a series of evaluation trials undertaken in the scope of the ACTS project AWACS in order to investigate the feasibility of the use of directional antennas to support high bit rate transmissions using a single carrier unequalised modem. The relationship between the antenna directivity, propagation statistics and modem performance are reported here. The results illustrate the effectiveness of correctly aligned, high gain antennas in reducing the undesired effects of multipath propagation. Further, the transceiver trials indicate the need for balancing antenna directivity at both ends of the link.

The TSUNAMI (II) project is an investigation into the use of adaptive antenna technology. The field trial system has been developed for the purposes of investigating the feasibility of implementing such a system using a GSM-1800 base station system. The field trial system uses a digital baseband beamforming technique (DBF) which, like all DBF systems, relies on the accurate transfer of weighted signals to and from the antenna array elements. As such, an accurate and reliable calibration system is required to combat effects arch as temperature and humidity on the individual antenna array paths. In this paper, results are presented of studies made on the field trial system, with regard to the operational temperature variations over an extended period of time. Investigations are presented of the TSUNAMI (II) downlink calibration technique and the implications for future adaptive antenna array calibration systems.

Frequency hopping spread spectrum (FH-SS) has found a number of applications in both CDMA and TDMA cellular systems, wireless local loop, and wireless local area networks. The effect of FH-SS on mobile channel characteristics is evaluated. Employing propagation studies, statistical analysis and simulation models, it is shown that the frequency-hopped channel displays improved characteristics when compared to the non-hopped case. The short term fading statistics are improved, which can be exploited to provide an overall increase in quality-of-service. The short term statistics of the frequency-hopped channel are derived, enabling prediction of the performance of an FH system.

This paper presents the results of an investigation into the wideband propagation characteristics of four different indoor operational environments within the 5.2 GHz band using a swept time-delay cross-correlator (STDCC) channel sounder. For each environment the cumulative distribution of delay spread for both omni-directional and a 20 dB horn antenna are presented. It is shown that a suitably aligned narrowbeam antenna makes possible to achieve delay spread reduction through spatial filtering. The effects of moving people and different room transmission situations are also presented.

This paper presents a selection of wideband channel sounding measurements performed as part of the ACTS AWACS (ATM Wireless Access Communications System) project. The results were obtained for two different indoor operating environments (mainly in line-of-sight conditions) at a carrier frequency of 19.37 GHz. The paper discusses the configuration of the wideband channel sounder and its connection to the prototype ATM transmission system. Measurements concentrate on average power, RMS delay spread and K-factor for a number of different antenna configurations and beamwidths. Mixed antenna configurations are also analysed with a directive antenna at the base station and an omni antenna at the mobile station. In particular, the results demonstrate the improvement in K-factor and the reduction in RMS delay spread that can be achieved with correctly oriented high gain antennas.

A wideband channel sounder is used to measure the complex impulse response of the radio channel, using a swept time delay cross correlator method. The sounder uses a chipping rate of 100 MHz, and has a resolution of 10 ns, equivalent to a path length difference of 3 metres. Omnidirectional vertically polarised antennas are used for both the transmitter and receiver. Indoor measurements are made in the Engineering School building, which has a mixture of small offices, and large open plan lab spaces, together with long corridors. Outdoor measurements are done within the campus, a compact collection of buildings of irregular shape. Ray tracing simulations are performed for the same environments using the personal computer-based Mirocell Communication Simulator (MCS), marketed by EDX Engineering, Inc., Oregon, USA. This simulator makes a number of simplifying assumptions, such as working only in two dimensions, and only allowing a one wall transmission per ray. Simplifying assumptions must also be made for the dielectric properties of the wall building materials. This paper compares simulated delay spreads with the measured values, in a variety of locations. An indoor measurement was done for a single receiver location within a large lecture room, with the transmitter in a nearby corridor. A series of indoor measurements was also done with the transmitter in a single location in a side corridor and the receiver moving along a route following a main corridor.

Spread spectrum (SS) techniques seem to be an attractive alternative to conventional narrow-band modulation. In this paper a frequency hopping (FH) system model based on slow FH (SFH) is proposed. The hopping pattern is in a pseudo-random manner through a set of completely independent channels. The simulated results of the bit error rate (BER) performance of the system for /spl pi//4 DQPSK modulation in the presence of additive white Gaussian noise (AWGN) and Rayleigh fading, together with Bose-Chaudhuri-Hocquenghem (BCH) coding and interleaving are obtained. Consideration is given to the optimum interleaving size, and the system performance in presence of both synchronous and asynchronous interferers.

This paper considers the performance of a DS-CDMA system which employs adaptive antenna technology at the basestation site. By utilising the capability of ray-tracing to provide the complex channel impulse response, a new ray-based simulation methodology for an adaptive antenna in a DS-CDMA system is presented. Results for a typical microcellular environment highlight the behaviour of the adaptive antenna. Finally, with the help of a DS-CDMA capacity analysis, the potential spectrum efficiency enhancement is evaluated and the improvement of some of the statistics of the system is discussed.

The use of adaptive antenna in mobile communications base stations is self-explanatory. It offers the potential of increased spectrum efficiency, extended range of coverage and reduced delay spread. However, one major concern for adaptive antenna system is the non-linearity effects of the RF/IF chain upon digital baseband beamforming network. This contribution investigates these effects on the performance of adaptive antennas and suggests the use of linearised transceiver system to tackle the nonlinearity problem.

Standardisation bodies in Europe and North America are currently developing standards for high data rate packet radio LANs. Radio LANs will have two main applications: wired LAN replacement and ad-hoc networking. In contrast to wired LAN replacement, ad-hoc networks will operate without control from a central node. As a consequence, no central synchronisation is provided and nodes must have the capability of acquiring all synchronisation from a received packet. This synchronisation can be achieved by matched filtering synchronisation codes inserted into each data packet. This paper discusses these synchronisation requirements and outlines synchronisation sequence selection criteria.>

Work at Bristol has considered the application of adaptive antenna technology to a mixed cell architecture employing DS-CDMA as the air interface. In the scenario considered, both the umbrella cells and the underlying micro cells operate within the same RF bandwidth in an attempt to support a seamless handover between different cell types. The paper presents an overview of the proposed mixed cell architecture employing adaptive antenna technology at the umbrella cell site, as well as the benefits that can be obtained from such a deployment. This is expressed in terms of the critical near-far problem and the system's capacity. Simulation and propagation results provide evidence for the claimed capabilities of the proposed system and estimates of the likely increased spectrum efficiency.>