In this paper, a decentralized wireless network based on OFDMA with multiuser diversity allocation has been modelled and simulated. Two different decentralized subcarrier allocation schemes have been presented, which are the collision subcarrier detection scheme and the incremental subcarrier allocation scheme. The convergence time of these decentralized subcarrier allocation schemes with and without Multiuser Diversity Allocation (MUDA) has been investigated, while their performance comparison with the centralized multiuser diversity allocation in terms of channel gains and channel capacity also has been performed. When the system with MUDA is compared to one without, results show a channel capacity improvement varying from 21% to 49% depending upon the number of subcarriers per transmitter. This is obtained at the cost of 0.3% to 23% more convergence time.

MIMO-OFDM with a massive number of transmit antennas (Massive MIMO-OFDM) promises to increase the spectrum efficiency or reduce the transmission energy per bit. The performance of Massive MIMO-OFDM is strongly influenced by the method used to estimate the channel state information (CSI) at the transmitter. Given the massive number of transmit antennas, the many training frames needed for CSI estimation decreases MAC efficiency and increases the cost of estimating CSI at a user station (STA). This paper presents a CSI estimation scheme that reduces the training frame length by using a rank enhancement pilot design. This design assigns a different CSI estimation weight to each subcarrier. The STAs feed unitary matrices, that are obtained by multiplying the left and right singular vectors, back to the AP. The proposed CSI method enables the AP to obtain accurate CSI for limited signal spaces and less-accurate CSI for wider signal spaces for setting data transmission weights and CSI estimation weights, respectively. Simulations of an IEEE802.11n channel show that the proposed CSI estimation scheme with very short training frames obtains greater than 95% of the achievable bit rate of a full CSI estimation in Massive MIMO-OFDM systems with 32 transmit antennas, 2 receive antennas, and 3 STAs.

This paper evaluates the impact of antenna patterns in heterogeneous LTE-Advanced networks. The propagation channel between each BS and UE is modelled as the spatial and polarimetric convolution of the antenna patterns with the spatial and temporal multipath components from a 3D outdoor ray-tracer. An LTE-Advanced physical layer simulator is used to determine the user performance in pico-cells and macro-cells for non-line-of-sight links. The received bit mutual information rate abstraction method is employed to determine the optimal physical layer throughput based on analysis of all available modulation and coding schemes. Packet error rate, throughput, link correlation and channel capacity are studied for pico-cellular and macro-cellular users. Results show that measured antenna patterns along with their orientations strongly influence user performance in an LTE-A network.

This paper presents a high-performance and biophysically accurate neuroprocessor architecture based on floating point arithmetic and compartmental modeling. It aims to overcome the limitations of traditional hardware neuron models that simplify the required arithmetic using fixed-point models. This can result in arbitrary loss of precision due to rounding errors and data truncation. On the other hand, a neuroprocessor based on a floating-point bio-inspired model, such as the one presented in this work, is able to capture additional cell properties and accurately mimic cellular behaviors required in many neuroscience experiments. The architecture is prototyped in reconfigurable logic obtaining a flexible and adaptable cell and network structure together with real time performance by using the available floating point hardware resources in parallel. The paper also demonstrates model scalability by combining the basic processor components that describe the soma, dendrite and synapse of organic cells to form more complex neuron structures.

In this paper, we propose the pilot design and channel estimation schemes for uplink block-spread code division multiple access (BS-CDMA) systems. In particular, we propose the use of a common spreading code to spread the pilot signals of all users such that a high bandwidth efficiency is achieved. The expense, however, is the multi-user interference (MUI) introduced in uplink channel estimation. On that account, three pilot design schemes are proposed to eliminate the MUI. We show through simulations that, a BS-CDMA system using the proposed pilot design schemes with recursive least squares (RLS) channel tracking algorithm achieves a comparable performance to a similar system using the pilot block based channel estimation (as employed in the LTE uplink) in low mobility scenario, while significantly outperforming that using the pilot block based channel estimation in high mobility scenario. In particular, the performance of BS-CDMA using the proposed channel estimation methods remains robust at a Doppler frequency of 400Hz.

The papers in this special issue focus on research and field trials of unmanned autonomous vehicles on land, in the air and underwater. The suite of selected papers covers key challenges that impact on the communications dynamics and behaviour of unmanned autonomous vehicles of varying size and resource capability and address UAV bridging, topology maintenance,path planning and link performance optimisation in highly changeable deployments.

The performance of an OFDM system can be improved by performing per-subcarrier antenna selection, which facilitates the exploitation of frequency and spatial diversity in the wireless channel. In this paper, we extend the concept of per-subcarrier antenna selection to a multiuser cognitive radio environment and present a practical subcarrier and antenna selection algorithm that exploits the multiuser, frequency and spatial diversities inherent in such systems while requiring only limited channel knowledge. The problem is formulated as an integer programming (IP) problem. We demonstrate that a linear relaxation of the problem still leads to an optimal solution, thus reducing the computational complexity relative to other approaches found in the literature. Simulation results demonstrate that the proposed resource allocation problem leads to an improvement in secondary users' link qualities, compared to a single-input single-output system and, at the same time, limits the interference to the primary user.

To facilitate coherent detection of orthogonal frequency division multiplexed transmissions, pilot symbols can be transmitted in some of the subcarriers. Their placement to minimise the mean squared error in channel estimation is considered. Such problems have been widely addressed previously for channels subject to wide sense stationary uncorrelated scattering processes. While the stationarity assumption is usually realistic, indoor wideband channels, for example, are subject to significantly correlated scattering processes. Furthermore, practical multiple antenna transmitters/receivers are bound to exhibit spatial correlations. This work considers pilot placement optimisation in a multiple antenna setting subject to arbitrary correlations in the delay and spatial domains. Noting that the naive approach incurs a prohibitive complexity, low complexity greedy solutions are developed. It is further proven that the conventional equi-spaced placement becomes optimal at high SNR even with correlations, when the correlation matrices are of full rank. A measurement campaign is conducted to estimate real-world channel statistics in an indoor wideband environment, and observe the applicability of developed algorithms. It is confirmed that practical channels are correlated to the extent of exhibiting rank deficiencies, and that the proposed placement algorithms result in improved channel estimation performance.

Low Density Parity Check (LDPC) codes have been adopted in a number of wired and wireless communication standards due to their improved error correcting ability and relatively simple decoder structure. However, for very high throughput systems operating in the multi-Gb/s range conventional decoding methods based on message passing are limited, due largely to the sheer volume of messages being exchanged. Thus, simpler decoding methods have been proposed such as bit flipping permitting efficient and fast hardware implementation. This paper presents two new bit flipping algorithm designed to reduce latency and power consumption. For a small loss in bit error rate performance (0.5 dB) we show how the application of an early stopping criteria uses 89% fewer iterations compared to a similar published algorithm. We also present a method for reducing power consumption by placing processing elements into a quiescent state based on a bit-local metric. Using this technique we show a potential reduction in power consumption of 76%.

In a downlink transmission, users can benefit from the high capacity gain achieved by transmitting independent data streams from multiple antennas to multiple users sharing the same physical time-frequency resources. This technique is known as multiuser MIMO (MU-MIMO). However, performance of MU-MIMO is sensitive towards propagation imperfections, such as time dispersion and inter-stream interference due to antenna correlation. In this paper we investigate the performance of MU-MIMO operation in 3GPP-LTE downlink transmission by employing a novel multiuser allocation, known as dynamic subcarrier allocation (DSA) that aims to minimize the effect of antenna correlation from the knowledge of the Effective Signal-to-Interference-plus-Noise Ratio (ESINR) metric. The suboptimal scheme is especially attractive when the number of users is large in highly correlated channel conditions where it is able to minimize the effect of spatial correlation and thereby achieve improved BER performance.

This paper aims to combine adaptive subcarrier allocation and bit loading with the transmission of the H.264 SVC (Scalable Video Coding) encoded video sequences in order to increase the number of supported users in the system and provide the best quality of service (QoS) to the subscribers. We initially assume that the number of calls at the base station can be supported, and present an integer program (IP) formulation of the problem that considers the frequency selective nature of the channel, bit error rate requirement and the discrete rate requirements of the different layers of the medium grain scalable (MGS) video. It is shown how the IP can be extended to perform call admission control (CAC). Due to the complexity involved with IP, a sub-optimal scheme is then presented. Results demonstrate that our proposed scheme performs better than systems with a fixed resource allocation strategy by supporting more users and by always achieving acceptable QoS. Furthermore, the low complexity of the proposed CAC schemes makes it suitable for practical application.

Biophysically accurate neuron models have emerged as a very useful tool for neuroscience research. These models are based on solving differential equations that govern membrane potentials and spike generation. The level of detail that needs to be presented in the model to accurately emulate the behaviour of an organic cell is still an open question, although the timing of the spikes is considered to convey essential information. Models targeting hardware are traditionally based on fixed point implementations and low precision algorithms which incur a significant loss of information. This, in turn, could affect the functionality of a bioelectronic neuroprocessor in an undefined way. In this paper, a 32-bit floating point reconfigurable somatic neuroprocessor is presented targeting an FPGA device for real-time processing. For each individual neuron, the dynamics of ionic channels are described by a set of first order kinetic equations. A dedicated CORDIC unit is developed to solve the nonlinear functions that regulate spike generation. The results have been verified using an experimental setup that combines an FPGA device and a digital-to-analogue converter.

For a single user with multiple receive antennas, co-antenna interference can be the dominant source of impairment in a Spatial Multiplex Orthogonal Frequency Division Multiple Access (SM-OFDMA) system, particularly when correlation exists between spatial sub-channels. By exploiting knowledge of the channel response and combining it with Dynamic Sub- Carrier Allocation (DSA), the algorithm proposed in this paper aims to reduce the effect of interference, while providing fair gain and maximizing the SINR across all users. Simulation results reveal that the proposed algorithm considers self-interference that exists between multiple antennas and is capable of achieving a capacity gain for every user with improved BER performance compared to previous work.

For the radar spectrum to be shared efficiently a good sensing capability within a secondary cognitive communication system is required. In this paper, the swept radar's rotation mechanism is explored to improve the sensing performance. Several node teaming algorithms are proposed for cooperative sensing along with the use of weighted sensing algorithms in a swept radar scenario. These teaming algorithms are considered in respect of the mobile team node selection and the sensing task assignments of the team nodes. Performance results show that selecting appropriate sensing nodes to join the sensing-active team in different sensing cycles and exploring their frequency diversity (to perform the sensing task at the most suitable frequency subchannels), yields a substantial improvement in performance. In addition, it is illustrated that proper node teaming algorithms should be chosen based on several key factors, including the characteristics of the primary signal and the sensing team node's computational capabilities.

This paper presents performance analysis of layered random beamforming (LRB) - MIMO-OFDMA employing various resource scheduling algorithms with feedback reduction in a realistic outdoor environment. LRB enables the exploitation of spatial multi-user diversity gain, spatial multiplexing capacity gain and layer spatial multi-user diversity gain, which is achieved by enabling the multiplex of data transmitted simultaneously to different destinations. Unlike a conventional beamforming system, an LRB system only requires an effective signal to interference and noise ratio (ESINR) based numerical data rate as feedback from every spatial layer of the MIMO channel and thus has potentially lower feedback requirements than a system which requires feedback of more detailed channel information. By combining the LRB technique with OFDMA, LRB-OFDMA can achieve an additional spectral multi-user diversity gain compared to the single carrier LRB system. Various scheduling algorithms are proposed for LRB-OFDMA and they show a trade-off between maintaining fairness and minimising delay. The performance of LRB-OFDMA is evaluated using some well-established statistical channel models as well as a propagation modeling tool, which represents a realistic outdoor environment.

The inefficiency of contemporary power amplifiers (PAs), when operating in their linear region, is a major obstacle to mobile operation of wireless local area networks (WLANs) based on IEEE 802.11n. Therefore the use of more efficient envelope elimination and restoration (EER) transmitter architectures is considered. In addition to high efficiency it is also necessary to satisfy the spectral mask and achieve satisfactory link-level performance. Link-level simulations of a contemporary WLAN PA show that, at the power back-offs necessary to achieve sufficient linearity, the power added efficiency (PAE) is only ∼1% for a system with four transmit antennas. In contrast, simulations of a phase feedback EER PA architecture show that it is possible to achieve an average PAE of 70%, while satisfying the spectral mask, with only a small degradation in link-level performance.

The feasibility of coexistence between radar and a communication system is investigated under log-normal shadowing conditions. A key element that enables coexistence is a sensing receiver to detect the presence of a radar system and prevent transmissions when interference may occur. Initial studies where each communication terminal does its own sensing and decision-making shows that while bandsharing is possible, under stringent interference requirements, communication efficiency can be limited. Analytical and simulation results illustrate that cooperative sensing leads to an improved detection range, higher detection probability and lower complexity of sensing nodes than that of single terminal sensing. It is concluded that cooperative spectrum sensing is able to enhance detection capabilities to maximise radar spectrum-sharing usage efficiency.

This paper presents a downlink performance analysis of a layered random beamforming (LRB) - MIMO- OFDMA physical layer (PHY) with feedback reduction as applicable to future generation wireless communication systems. OFDMA is a popular multiple access candidate for future generation cellular communication systems which facilitates multi-user diversity by enabling multiple access in the frequency domain. LRB enables the exploitation of spatial multi-user diversity gain, spatial multiplexing capacity gain and layer spatial multi-user diversity gain, which is achieved by enabling the multiplex of data transmitted simultaneously to different destinations. Unlike a conventional beamforming system, an LRB system only requires effective signal to interference and noise ratios (ESINR) as feedback from every spatial layer of the MIMO channels and thus has potentially lower feedback requirements than a system which requires feedback of more detailed channel information. By combining the LRB technique with OFDMA, LRB-OFDMA can achieve an additional spectral multi-user diversity gain compared to the single carrier LRB system. However, in this case ESINR feedback on a per-sub-carrier basis is required in principle and the feedback requirements may thus increase substantially. This feedback requirement can be reduced by generating the feedback information on a cluster (group of sub-carriers) basis rather than on an individual sub-carrier basis. In this way, the system can exploit any correlation in the frequency response of the channel. The design of an LRB- OFDMA system is presented in this paper and the performance of the system is evaluated for different degrees of feedback reduction using various statistical channel models. A complete list of feedback requirements for various MIMO schemes is also presented.

Over the years peer-to-peer (P2P) multi-hop relays have been studied for mobile ad-hoc networks (MANETs) to overcome poor signal coverage and to improve connectivity. However, for an urban environment, particularly with a sparse mobile distribution, a ground relay is not able to achieve significant benefits, mainly due to the extremely high pathloss and shadowing encountered. Dedicated airborne relay nodes, at a height of hundreds or thousands of meters, can provide much better coverage, and hence improve the connectivity, decrease the number of hops, reduce the power consumption, and support co-operative relays when integrated into mobile ad-hoc networks. An air node can also act as the sink for current wireless sensor networks (WSNs). Despite these potential advantages, the coverage from an air node located directly above an urban operating environment has not been adequately investigated in the literature. This paper compares the advantages of airborne relaying compared to more conventional peer-to-peer mobile relays. Three channel types are considered, i.e. line-of-sight (LoS), obstructed LoS (OLoS), and non-LoS (NLoS), each with their own likelihood, pathloss, and shadowing models. The practical gain pattern of a hemispheric antenna, or a directional antenna, is also carefully considered for the air node. Comparisons of the air-to-ground (A2G) channel and the mobile P2P channel demonstrate that airborne relays can enhance the coverage and reduce the power consumption significantly.

This paper presents a downlink performance analysis of a layered random beamforming (LRB) - MIMO-OFDMA physical layer (PHY) as applicable to future generation wireless communication systems. OFDMA is a popular multiple access candidate for future generation cellular communication systems which facilitates multi-user diversity by enabling multiple access in the frequency domain. Random beamforming (RB) is a method which enables the exploitation of spatial multi-user diversity gain and a spatial multiplexing capacity gain. Unlike a conventional beamforming system, an RB system only requires effective signal to noise ratios (ESNR) as feedback and thus has potentially much lower feedback requirements than a system which requires feedback of more detailed channel information. The layered method (LRB) enables the spatial multiplex of data transmitted simultaneously to different destinations. As a result it is able to offer a further spatial multi-user diversity gain, referred to here as layer multi-user diversity gain to distinguish from that achieved by the RB technique. An LRB-OFDMA system is thus able to achieve the rich benefits of spatial multiplexing capacity gain in combination with spatial, layer and spectral multi-user diversity gains. In this paper, the design of both RB-OFDMA and LRB-OFDMA systems are proposed and the performance of the system is evaluated by software simulation using various statistical channel models

The likelihood of line-of-sight (LoS) is an essential component in any radio channel model. It is particularly useful for radio network planning and urban coverage prediction. Empirical LoS models are hard to derive due to a strong dependency on local topology and the need for large measurement datasets. Since buildings are the major obstructions in a dense urban environment, we propose a new theoretical model to determine the LoS probability for air-to- ground channels based on local building geometry and knife-edge diffraction theory. The model takes into account key statistical parameters such as building height, building size, building coverage, street width and street angle distribution. The theoretical model is shown to agree well with ray tracing simulation results. The statistical parameters, such as mean building height, percentage of area covered by buildings (building coverage) and building density, can all be easily obtained for a specific location. We also derive equations for the likelihood of LoS for a direct slant path. These equations can be used in the analysis of air-to-ground channels.

This paper provides new statistical models for air-to-ground channels in an urban environment. The model is derived to operate at frequencies from 200MHz to 5GHz. Issues such as path loss and shadowing are evaluated as a function of the elevation angle to the airborne platform, rather than the more usual separation distance used for terrestrial mobile communications. Results demonstrate the advantages of an air-to-ground channel for urban communication, and relayed peer-to-peer links in particular.

Relaying is a known method used to increase coverage in wireless communication systems. In addition to coverage increase, new relaying methods have been proposed that may increase the multi-hop wireless system capacity by taking advantage of higher data rates achievable with intermediate relaying nodes. At the same time, multi-hop wireless networks can be created, relatively simply, by running any of the proposed MANET routing protocols. Several questions can be raised: what is the best achievable network performance for a system composed of single-frequency wireless nodes able to use cooperative and non-cooperative relaying? how does the network throughput of a system using relaying nodes compare to that of a system that does not use relaying at all? This paper presents initial answers to these questions using results obtained through the simulation of single-frequency wireless ad-hoc networks using static and dynamic MANET routing, with nodes supporting several co-operative relaying schemas

This paper presents a downlink performance analysis of a link adaptation (LA) algorithm applied to a MIMO-OFDM physical layer (PHY) which is a popular candidate for future generation cellular communication systems. The new LA algorithm attempts to maximize throughput and adaptation between various modulation and coding schemes in combination with both space-time block codes (STBC) and spatial multiplexing (SM) is based on knowledge of SNR and H matrix determinant; the parameters which are found to have the most significant influences on the system throughput. By means of ray-tracing software, a detailed example coherent time-variant channel trace is generated. Subsequent application of physical layer simulation software to this channel trace yields throughput results for adaptive MIMO-OFDM for the cases of the proposed LA algorithm, an algorithm based on SNR only, an algorithm based on SNR and H matrix determinant with fixed threshold levels for different SM modes and the optimal case.

This paper presents a downlink performance evaluation of a candidate physical layer for future generation cellular communications systems employing link adaptive MIMO-OFDMA. Adaptation between various modulation and coding schemes in combination with both space-time block codes (STBC) and spatial multiplexing (SM) on the basis of channel knowledge is considered. By means of ray-tracing software, a detailed example coherent time-variant channel trace is generated. Subsequent application of physical layer simulation software to this channel trace yields PER and throughput results for all available link adaptation parameters. Various channel parameters are considered against this data and the SNR and H matrix determinant are found to have the most significant influences on the system throughput. Subsequently, LA algorithms are proposed whereby the system adapts according to SNR only and both SNR and H matrix determinant. The performance of these algorithms is compared to the optimal case.

Relaying is a known method for increasing coverage in wireless communication systems. In addition to coverage increase, new cooperative relaying methods have been proposed that may increase the wireless system capacity by taking advantage of higher data rates in intermediate cooperating relaying nodes. This paper investigates the performance of several of these relaying methods in terms of maximum achievable throughput at MAC service access point for single-frequency wireless ad-hoc networks. The simulation framework used is described, and the performance limitations of the relaying methods are analysed. We find that relaying, cooperative or not, does not significantly increase maximum achievable throughput for line-of-sight (LOS) propagation environments. The achievable throughput can be significantly increased for non-line of sight (NLOS) environments under specific conditions for source/destination and intermediate relaying nodes

Dynamic sub-carrier allocation (DSA) strategies have been shown previously to achieve significant performance benefits when applied to OFDMA systems and further benefits for MIMO-OFDMA systems. Analysis thus far has focussed on the assumption of ideal channel state information (CSI). In this paper, the impact of non-ideal CSI is investigated. Various channel estimation techniques are evaluated for application to MIMO-OFDMA systems. They are based on least squares (LS) estimation with training pilots. `Conventional' (as for MIMO-OFDM) CTP (combining training pilots) and `improved' (optimised for MIMO-OFDMA) STP (separate training pilots) versions of both frequency domain least square (FDLS) and time domain least square (TDLS) channel estimation are considered, as are the options of both space-time block coding (STBC) and spatial multiplexing (SM) as MIMO strategies. The STP-TDLS strategy is shown to significantly outperform other channel estimation options, achieving performance within 1 dB of the ideal case. Subsequently, the use of the STP-TDLS channel estimation method in conjunction with the DSA algorithm is considered in order to determine the impact of non-ideal channel knowledge on the gain achieved by DSA. The performance for the cases of ideal CSI and CSI derived via STP-TDLS channel estimation are compared and evaluated for both the STBC and SM cases. The effects of non-ideal channel estimation in both the DSA mechanism and channel equalisation separately and together are evaluated. It is shown that STP-TDLS channel estimation works better in SM (only IdB worse than ideal CSI case) than in STBC. Furthermore, it is shown that DSA is less sensitive than channel equalisation to nonideal CSI. The degradation of system performance in the realistic case of non-ideal CSI for both DSA and channel equalisation is a compound of the effects of the separate effects of non-ideal CSI error. It is shown here that in both STBC and SM cases, the effect is almost a linear addition of the two parts. Given the substantial benefits of DSA and its relative insensitivity to channel estimation errors, it is concluded that DSA remains a highly promising technique

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

Medium access control (MAC) protocols have a large impact on the achievable system performance for wireless ad hoc networks. Because of the limitations of existing analytical models for ad hoc networks, many researchers have opted to study the impact of MAC protocols via discreteevent simulations. However, as the network scenarios, traffic patterns and physical layer techniques may change significantly, simulation alone is not efficient to get insights into the impacts of MAC protocols on system performance. In this paper, we analyze the performance of IEEE 802.11 distributed coordination function (DCF) in multihop network scenario. We are particularly interested in understanding how physical layer techniques may affect the MAC protocol performance. For this purpose, the features of interference range is studied and taken into account of the analytical model. Simulations with OPNET show the effectiveness of the proposed analytical approach.

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.

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.

In this paper an iterative channel estimation technique for a mobile fourth generation coherent orthogonal frequency division multiplexing (OFDM) system is proposed. Coherent detection dictates that a per-subband estimate of the frequency response of the channel is generated for each OFDM symbol. This is often achieved by inserting pilot symbols amongst the data symbols in the OFDM modulation grid and interpolating accordingly. This paper proposes a lower complexity channel estimation method that exploits coding gain to achieve a channel estimate that yields an overall system performance that is shown to be comparable to the scattered pilot system, but with only one initial training sequence. A tracking algorithm is proposed which improves performance at high mobile velocities.

The paper reports on a new class of equalisation/detection for wideband multiple input-multiple output (MIMO) communications systems. The proposed scheme is somewhat akin to a multi-carrier MIMO system, and more precisely, builds upon a single carrier frequency domain equalised (SCFDE) MIMO system. At the core of our system lies a novel concept of iterative self-reused equalisation/detection (bootstrapping) combined with semi-hard decision making. The bootstrapping concept is derived from a new formulation of Tikhonov regularisation. The proposed scheme achieves a remarkable performance/complexity trade-off. In particular we show that our system approaches the performance of ML-MIMO-OFDM while being only slightly more complex than MMSE-MIMO-OFDM. The coded version achieves virtually the same performance as turbo MIMO-OFDM at a fraction of the complexity. The resulting system is termed zero tailed bootstrap frequency equalised (ZTBFE) MIMO system.

The paper considers a multiple-input multiple-output (MIMO) communication system, which uses space-time trellis coding (STTC). A novel method of decoding STTC without a need to transmit training sequences is developed. The technique uses only a single channel estimate to acquire a complete set of the channels' estimates while performing STTC detection. The method is akin to blind trellis search techniques (per-survivor processing - PSP) and adaptive Viterbi. Our solution consists of the deployment of a bank of Kalman filters. The bank of Kalman filters is coupled with Viterbi type decoders, which produce tentative decisions based on Kalman channel predictions. In return, the Kalman filters use the tentative decisions to update and track the MIMO channels corresponding to a number of tracked hypotheses. The proposed technique is particularly applicable to space-time systems operating in rapidly fading environments, where STTC can be decoded and MIMO channels efficiently tracked without relying on periodic pilot sequences.

This paper considers an OFDM system with a multiple-input multiple-output (MIMO) configuration, which uses space-frequency trellis coding (SFTC). A novel method of decoding SFTC without a need to transmit separate training sequences is developed. The technique uses only a single frequency tone to acquire a complete set of the channels' estimates while performing SFTC detection. The method is akin to blind trellis search techniques (per-survivor processing - PSP) and adaptive Viterbi. Our solution consists of the deployment of a bank of Kalman filters. The bank of Kalman filters is coupled with Viterbi type decoders, which produce tentative decisions based on Kalman channel predictions. In return, the Kalman filters use the tentative decisions to update and track the MIMO channels corresponding to a number of tracked hypotheses.

This paper describes a newly developed three dimensional ray launching model for complex indoor environments. The model is capable of predicting detailed temporal and spatial multipath. To investigate and evaluate the accuracy of this model, predictions are compared with detailed measurements taken in an office environment. Power delay and power azimuth spectra are measured using a multielement Medav channel sounder operating with 120 MHz of bandwidth in the 5 GHz band. Prediction accuracy was found to be sensitive to the accuracy of the basestation height, the permittivity of the surrounding materials and the number of reflections permitted in the model. Overall, the results indicate that with careful modelling and well calibrated measurements, good site specific spatial and temporal multipath agreement can be achieved.

We examine the performance of a space-time coded (STC) OFDM system. As an example, two emerging WLAN standards (IEEE 802.11a and ETSI HIPERLAN/2) are modified to accommodate the STC technique. Performance results using statistical multi-element channel models are compared with those using state-of-the-art indoor MIMO channel measurements. We conclude that the gap between the ideal setting and our measured channel is 4, 5 and 6 dB for 1, 2 and 3 receive antennas respectively. We observe also that STC with (2Tx, 3Rx) over the measured channels doubles the capacity as compared to the equivalent mode of HIPERLAN/2.

In order to evaluate the performance of third-generation mobile communication systems, radio channel models are required. The models should be capable of handling nonstationary scenarios with dynamic evolution of multipath. In this context and due to the introduction of advanced antenna systems to exploit the spatial domain, a further expansion is needed in order to include the nonstationary characteristics of the channel. In an attempt to solve these problems, this paper presents a new stochastic spatio-temporal propagation model. The model is a combination of the geometrically-based single reflection and the Gaussian wide-sense stationary uncorrelated scattering models, and is further enhanced in order to be able to handle nonstationary scenarios. The probability density functions of the number of multipath components, the scatterers' lifetime, and the angle of arrival are calculated to support these features. The input parameters of the model are based on results from measurement campaigns published in the open literature.

This paper compares the performance of pilot tone aided (PTA) and pilot symbol aided (PSA) BPSK, QPSK, 8QAM, 16QAM, 32QAM and 64QAM over a narrowband radio channel. This set of modems is capable of achieving 14, 28, 48, 56, 70 and 84 kbit/s respectively in a 20 kHz channel bandwidth. Computer simulations in a Rayleigh fading channel show that PTA modems have almost the same performance as PSA modems. However PTA signals have a higher peak to mean power ratio (/spl eta/) and this translates into greater amplifier linearity requirements in the transmitter. In the PSA modems, /spl eta/ is independent of the pilot symbols, however in the PTA modems it depends upon the pilot tone to data power ratio (R). An optimisation of R in an AWGN channel gives a value of 0.2 for QAM. A comparison of /spl eta/ between PSA and PTA transmitted signals show differences of 3.38, 5.1, 4.8, 4.25, 4.45 and 3.9 dB for 99.9% of the time (based on the above mentioned list of modulation schemes).

Due to the site specific nature of microcellular operational environments, propagation models are required to take into account the exact position, orientation and electrical properties of individual buildings, and hence, ray-tracing techniques have emerged as the dominant methods to predict propagation in such environments. A novel hybrid three-dimensional (3-D) ray-tracing algorithm which can evaluate scenarios incorporating many thousands of objects by utilising the concept of "illumination zones," is presented. In order to evaluate the accuracy of the presented model, comparisons of narrow-band and wide-band predictions with measurements are performed for a variety of scenarios. First, power comparisons show that very accurate predictions can be achieved (RMS errors less than 3.7 dB). Then, wide-band analysis shows that since the RMS delay spread for systems with finite bandwidth is a function of the multipath phase, only average measured and predicted RMS delay spread values can be compared and as a result, limited averaging can produce large RMS errors. With sufficient averaging the achieved wide-band accuracy in terms of the predicted RMS delay spread, is adequate for most planning purposes.

A three dimensional `image-based' ray tracing algorithm for indoor environments is presented. The model is capable of predicting wideband as well as narrowband propagation information for single floor scenarios. To show the practicality of such a model, the ray tracing engine has been used to investigate a typical indoor environment. The influence of internal objects such as windows, doors and partitions is considered. Since externally reflected and diffracted rays are supported, the impact of external building structures on the received power and RMS delay spread is examined. The channel characteristics were also studied for different receiver antenna orientations.

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.

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.

An important parameter in characterizing radio communications channels is the coherence bandwidth. This paper presents an analysis of the coherence bandwidth in a urban microcell environment where the dynamic channel response is determined by a site-specific ray-tracing propagation model. Such an analytical model provides a direct calculation of signal fading envelope correlation as a function of frequency and location. The analysis here shows that coherence bandwidth is strongly dependent on location within a particular propagation environment and only weakly related to RMS delay spread. Typical results for frequency diversity gain for various frequency separations are also presented.

An important improvement in the performance of CDMA and other wideband digital communication systems can be achieved using a RAKE receiver architecture and diversity combining of its branches. The paper presents an analysis of RAKE branch statistics as a function of chipping or channel symbol transmission rate for urban microcells. A deterministic ray-tracing model is used to provide realistic, site-specific estimates of the time dispersion in the dynamic fading channel. The results of the analysis show that increased chipping rates produce increasingly Ricean fading distributions on the diversity branches along with increased power in the higher order diversity branches. Increased diversity branch power results in more effective diversity combining and improved signal-to-noise ratios of the combined branch signals.

The problem of choosing the optimum locations for base stations in a microcell communications system is considered. The factors affecting optimum selection are the non-uniform service areas in complex propagation environments, the mutual coverage and interference of multiple base stations, and the service objective which is usually defined by signal level but may also be defined by delay spread for certain digital systems. This optimization problem is addressed using simulated annealing (SA) techniques which offer a method to achieve near optimum solutions to complex combinatorial optimization problems, This research shows that SA is a viable approach for handling practical base station siting problems in an urban microcell environment.>

A sliding window decorrelating algorithm is developed. It is shown to be near-far resistant both in AWGN and fast fading environments. The algorithm is of particular relevance to base stations in multi-user DS-CDMA mobile radio networks, and is shown to alleviate the requirement for a complex power control algorithm to compensate for rapidly varying relative user energies.>

In this contribution, a software multipath fading simulation package (previously applied to the statistical analysis of the fading statistics of single transmitter schemes for three very different mobile radio environments) is used to analyze the performance of a quasi-synchronous, multitransmitter, amplitude modulated sideband diversity area coverage system. By conducting field trials in an area known to obey Rayleigh fading statistics, it has been possible to compare the simulated statistical results to those obtained theoretically and experimentally, and to show that good agreement exists. The fading simulator is capable of being extended to other types of multitransmitter radio scheme employing different forms of modulation.

The paper describes a new pilot tone SSB configuration, transparent tone-in-band (TTIB), which may be used in mobile radio systems from low-band VHF to microwave frequencies. By utilizing audio signal processing techniques in the transmitter and receiver, the pilot reference tone may be positioned centrally within the RF channel bandwidth without losing the property of data transparency and also retains the many system advantages of tone-in-band SSB over the pilot carrier and tone-above-band schemes. Besides speech transmissions, results are presented for noncoherent FSK and DPSK data formats under white noise and Rayleigh fading conditions. Finally, a new technique utilizing TTIB is suggested to facilitate the use of coherent data systems.

Although long-loop phase-locked techniques are finding increasing application in the fields of mobile radio and satellite communications, their use has several drawbacks. Time delay associated with filtering processes within the loop gives rise to a degradation in acquisition capability, and information components entering the control loop cause a reduction in adjacent channel performance. In this paper, a split-loop technique is described, which eliminates false-locking, substantially improves the acquisition characteristics to that associated with a loop with zero time delay, and "cleans up" the first local oscillator spectrum. Furthermore, the technique is simple to implement.